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How to change the color of a selected item of a ComboboxItem which includes a TextBlock?

I have the following XAML of a Combobox:
<ComboBox>
<ComboBoxItem>
<TextBlock FontWeight="Bold" Background="PaleGreen">1</TextBlock>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock FontWeight="Bold" Background="Salmon">2</TextBlock>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock FontWeight="Bold" Background="PaleTurquoise">3</TextBlock>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock FontWeight="Bold" Background="Gold">4</TextBlock>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock FontWeight="Bold" Background="Black" Foreground="White">5</TextBlock>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock FontWeight="Bold" Background="Gray">6</TextBlock>
</ComboBoxItem>
</ComboBox>
The following pictures shows my problem:
Combobox open
Combobox closed
So you can see in the picture "Combobox closed" that the number "4" has no color, but I want that the text "4" is yellow. Can you help me?

#janonimus: When I add a template as you said it has following code:
<Window.Resources>
<ControlTemplate x:Key="ComboBoxControlTemplate1" TargetType="{x:Type ComboBox}">
<Grid x:Name="Placement" SnapsToDevicePixels="True">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Popup x:Name="PART_Popup" AllowsTransparency="True" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
<Themes:SystemDropShadowChrome x:Name="Shdw" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=Placement}">
<Border x:Name="DropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
<ScrollViewer x:Name="DropDownScrollViewer">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=DropDownBorder}" Height="{Binding ActualHeight, ElementName=DropDownBorder}" Width="{Binding ActualWidth, ElementName=DropDownBorder}"/>
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Grid>
</ScrollViewer>
</Border>
</Themes:SystemDropShadowChrome>
</Popup>
<Themes:ListBoxChrome x:Name="Border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" RenderMouseOver="{TemplateBinding IsMouseOver}" RenderFocused="{TemplateBinding IsKeyboardFocusWithin}"/>
<TextBox x:Name="PART_EditableTextBox" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" IsReadOnly="{Binding IsReadOnly, RelativeSource={RelativeSource TemplatedParent}}" Margin="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}">
<TextBox.Style>
<Style TargetType="{x:Type TextBox}">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="AllowDrop" Value="True"/>
<Setter Property="MinWidth" Value="0"/>
<Setter Property="MinHeight" Value="0"/>
<Setter Property="FocusVisualStyle" Value="{x:Null}"/>
<Setter Property="ScrollViewer.PanningMode" Value="VerticalFirst"/>
<Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<ScrollViewer x:Name="PART_ContentHost" Background="Transparent" Focusable="False" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</TextBox.Style>
</TextBox>
<ToggleButton Grid.Column="1" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}">
<ToggleButton.Style>
<Style TargetType="{x:Type ToggleButton}">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="IsTabStop" Value="False"/>
<Setter Property="Focusable" Value="False"/>
<Setter Property="ClickMode" Value="Press"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Themes:ButtonChrome x:Name="Chrome" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" RenderMouseOver="{TemplateBinding IsMouseOver}" RenderPressed="{TemplateBinding IsPressed}" RoundCorners="False" SnapsToDevicePixels="True" Width="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}">
<Path x:Name="Arrow" Data="M0,0L3.5,4 7,0z" Fill="Black" HorizontalAlignment="Center" Margin="0,1,0,0" VerticalAlignment="Center"/>
</Themes:ButtonChrome>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter Property="RenderPressed" TargetName="Chrome" Value="True"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Fill" TargetName="Arrow" Value="#FFAFAFAF"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ToggleButton.Style>
</ToggleButton>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="IsDropDownOpen" Value="True">
<Setter Property="RenderFocused" TargetName="Border" Value="True"/>
</Trigger>
<Trigger Property="HasItems" Value="False">
<Setter Property="Height" TargetName="DropDownBorder" Value="95"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
<Setter Property="Background" Value="#FFF4F4F4"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsGrouping" Value="True"/>
<Condition>
<Condition.Value>
<System:Boolean>False</System:Boolean>
</Condition.Value>
</Condition>
</MultiTrigger.Conditions>
<Setter Property="ScrollViewer.CanContentScroll" Value="False"/>
</MultiTrigger>
<Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="True">
<Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
<Setter Property="Color" TargetName="Shdw" Value="#71000000"/>
</Trigger>
<Trigger Property="CanContentScroll" SourceName="DropDownScrollViewer" Value="False">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Window.Resources>
In addition, an error is thrown as soon as I have inserted the template:
InvalidOperationException: Nicht-NULL-Wert für "Property" erforderlich.
EDIT:
I have the solution. It was quite easy :D I don't need a template. I just added triggers:
<Style TargetType="ComboBox">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="FontSize" Value="10"/>
<Setter Property="HorizontalAlignment" Value="Center"/>
<Setter Property="VerticalAlignment" Value="Center"/>
<Setter Property="Width" Value="115"/>
<Setter Property="Foreground" Value="Black"/>
<Setter Property="BorderBrush" Value="White"/>
<Setter Property="SelectedIndex" Value="0"/>
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="IsEditable" Value="True"/>
<EventSetter Event="SelectionChanged" Handler="SelectionChanged"/>
<Style.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Opacity" Value="0.4"/>
</Trigger>
<Trigger Property="SelectedIndex" Value="0">
<Setter Property="Background" Value="PaleGreen"/>
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="SelectedIndex" Value="1">
<Setter Property="Background" Value="Salmon"/>
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="SelectedIndex" Value="2">
<Setter Property="Background" Value="PaleTurquoise"/>
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="SelectedIndex" Value="3">
<Setter Property="Background" Value="Gold"/>
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="SelectedIndex" Value="4">
<Setter Property="Background" Value="Black"/>
<Setter Property="Foreground" Value="White"/>
</Trigger>
<Trigger Property="SelectedIndex" Value="5">
<Setter Property="Background" Value="Gray"/>
<Setter Property="Foreground" Value="Black"/>
</Trigger>
</Style.Triggers>
</Style>

You need to edit the template of the ComboBox.
Go to [Document Outline], right-click the ComboBox, Edit Template->Edit a Copy.
Then find the "ComboBoxEditableTemplate" and edit the Border the contains the "PART_EditableTextBox". Here's the specific part of the generated code::
...
<ControlTemplate x:Key="ComboBoxEditableTemplate"
TargetType="{x:Type ComboBox}">
<Grid x:Name="templateRoot"
SnapsToDevicePixels="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="0"
MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" />
</Grid.ColumnDefinitions>
<Popup x:Name="PART_Popup"
Grid.ColumnSpan="2"
AllowsTransparency="true"
IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}"
Placement="Bottom"
PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}">
<Themes:SystemDropShadowChrome x:Name="shadow"
MinWidth="{Binding ActualWidth, ElementName=templateRoot}"
MaxHeight="{TemplateBinding MaxDropDownHeight}"
Color="Transparent">
<Border x:Name="dropDownBorder"
Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"
BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}"
BorderThickness="1">
<ScrollViewer x:Name="DropDownScrollViewer">
<Grid x:Name="grid"
RenderOptions.ClearTypeHint="Enabled">
<Canvas x:Name="canvas"
Width="0"
Height="0"
HorizontalAlignment="Left"
VerticalAlignment="Top">
<Rectangle x:Name="opaqueRect"
Width="{Binding ActualWidth, ElementName=dropDownBorder}"
Height="{Binding ActualHeight, ElementName=dropDownBorder}"
Fill="{Binding Background, ElementName=dropDownBorder}" />
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter"
KeyboardNavigation.DirectionalNavigation="Contained"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
</Grid>
</ScrollViewer>
</Border>
</Themes:SystemDropShadowChrome>
</Popup>
<ToggleButton x:Name="toggleButton"
Grid.ColumnSpan="2"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
Style="{StaticResource ComboBoxToggleButton}" />
<!-- EDIT THIS PART LIKE SO -->
<Border x:Name="border"
Margin="{TemplateBinding BorderThickness}"
Background="{Binding SelectedItem.Background, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}, FallbackValue=Transparent}">
<!-- Background="{StaticResource TextBox.Static.Background}"> -->
<!-- //////////////////////// -->
<TextBox x:Name="PART_EditableTextBox"
Margin="{TemplateBinding Padding}"
HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
IsReadOnly="{Binding IsReadOnly, RelativeSource={RelativeSource TemplatedParent}}"
Style="{StaticResource ComboBoxEditableTextBox}" />
</Border>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="false">
<Setter TargetName="border" Property="Opacity" Value="0.56" />
</Trigger>
<Trigger Property="IsKeyboardFocusWithin" Value="true">
<Setter Property="Foreground" Value="Black" />
</Trigger>
<Trigger SourceName="PART_Popup" Property="HasDropShadow" Value="true">
<Setter TargetName="shadow" Property="Margin" Value="0,0,5,5" />
<Setter TargetName="shadow" Property="Color" Value="#71000000" />
</Trigger>
<Trigger Property="HasItems" Value="false">
<Setter TargetName="dropDownBorder" Property="Height" Value="95" />
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsGrouping" Value="true" />
<Condition Property="VirtualizingPanel.IsVirtualizingWhenGrouping" Value="false" />
</MultiTrigger.Conditions>
<Setter Property="ScrollViewer.CanContentScroll" Value="false" />
</MultiTrigger>
<Trigger SourceName="DropDownScrollViewer" Property="ScrollViewer.CanContentScroll" Value="false">
<Setter TargetName="opaqueRect" Property="Canvas.Top" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}" />
<Setter TargetName="opaqueRect" Property="Canvas.Left" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
...
After that, you can use it like the following, without using TextBlocks:
<ComboBox IsEditable="True"
Style="{DynamicResource YourComboBoxStyleName}">
<ComboBoxItem Background="PaleGreen"
Content="1"
FontWeight="Bold" />
<ComboBoxItem Background="Salmon"
Content="2"
FontWeight="Bold" />
<ComboBoxItem Background="PaleTurquoise"
Content="3"
FontWeight="Bold" />
<ComboBoxItem Background="Gold"
Content="4"
FontWeight="Bold" />
<ComboBoxItem Background="Black"
Content="5"
FontWeight="Bold"
Foreground="White" />
<ComboBoxItem Background="Gray"
Content="6"
FontWeight="Bold" />
</ComboBox>

WPF How To Hide ComboBoxItem BorderBrush On MouseOver? (the blue selection rectangle)

I want to display an Imagebased Combobox in WPF. It works with some templating but I don't figure out how to get rid of the blueish mousehover rectangle.
See the light blue box:
OK I found the solution, the question is solved now See the solution down here:
The Solution:
<ComboBox x:Name="comboBox"
Width="158"
Height="44"
Background="Transparent"
BorderBrush="#551B2830"
Foreground="Black">
<ComboBoxItem TextBlock.TextAlignment="Center">Mousehover me</ComboBoxItem>
<ComboBoxItem TextBlock.TextAlignment="Center">To see the bad </ComboBoxItem>
<ComboBoxItem TextBlock.TextAlignment="Center">blue rectangle</ComboBoxItem>
<ComboBox.ItemContainerStyle>
<Style TargetType="ComboBoxItem">
<Setter Property="BorderBrush" Value="Transparent" />
<Setter Property="BorderThickness" Value="0" />
<Setter Property="Template">
<Setter.Value>
<!--This Does the Magic-->
<ControlTemplate TargetType="{x:Type ComboBoxItem}">
<Border x:Name="Bd"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
SnapsToDevicePixels="true">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
SnapsToDevicePixels="True" />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ComboBox.ItemContainerStyle>
</ComboBox>
</Grid>

Ok found out how to do it .. I can add a template for the comboboxItem without Border or with Invisible Border.
I also updated the question with the solution.
Edit: And here also to mark it as answered..
<ComboBox x:Name="comboBox"
Width="158"
Height="44"
Background="Transparent"
BorderBrush="#551B2830"
Foreground="Black">
<ComboBoxItem TextBlock.TextAlignment="Center">Mousehover me</ComboBoxItem>
<ComboBoxItem TextBlock.TextAlignment="Center">To see the bad </ComboBoxItem>
<ComboBoxItem TextBlock.TextAlignment="Center">blue rectangle</ComboBoxItem>
<ComboBox.ItemContainerStyle>
<Style TargetType="ComboBoxItem">
<Setter Property="BorderBrush" Value="Transparent" />
<Setter Property="BorderThickness" Value="0" />
<Setter Property="Template">
<Setter.Value>
<!--This Does the Magic-->
<ControlTemplate TargetType="{x:Type ComboBoxItem}">
<Border x:Name="Bd"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
SnapsToDevicePixels="true">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
SnapsToDevicePixels="True" />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ComboBox.ItemContainerStyle>
</ComboBox>

This will work for you :
<ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"KeyboardNavigation.DirectionalNavigation="Contained"/>
</ScrollViewer></Border>
</Microsoft_Windows_Themes:SystemDropShadowChrome></Popup>
<Microsoft_Windows_Themes:ListBoxChrome x:Name="Border" Grid.ColumnSpan="2" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" RenderFocused="{TemplateBinding IsKeyboardFocusWithin}" RenderMouseOver="{TemplateBinding IsMouseOver}"/>
<TextBox x:Name="PART_EditableTextBox" Margin="{TemplateBinding Padding}" Style="{StaticResource ComboBoxEditableTextBox}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" IsReadOnly="{Binding IsReadOnly, RelativeSource={RelativeSource TemplatedParent}}"/>
<ToggleButton Style="{StaticResource ComboBoxToggleButton}"Grid.Column="1" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsKeyboardFocusWithin" Value="true">
<Setter Property="Foreground" Value="Black"/></Trigger>
<Trigger Property="IsDropDownOpen" Value="true">
<Setter Property="RenderFocused" TargetName="Border" Value="true"/></Trigger>
<Trigger Property="HasItems" Value="false">
<Setter Property="Height" TargetName="DropDownBorder" Value="95"/></Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
<Setter Property="Background" Value="#FFF4F4F4"/></Trigger>
<Trigger Property="IsGrouping" Value="true">
<Setter Property="ScrollViewer.CanContentScroll" Value="false"/></Trigger>
<Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
<Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
<Setter Property="Color" TargetName="Shdw" Value="#71000000"/></Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Style x:Key="ComboBoxStyle1" TargetType="{x:Type ComboBox}">
<Setter Property="FocusVisualStyle" Value="{StaticResource ComboBoxFocusVisual}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}"/>
<Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
<Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="Padding" Value="4,3"/>
<Setter Property="Template"><Setter.Value>
<ControlTemplate TargetType="{x:Type ComboBox}">
<Grid x:Name="MainGrid" SnapsToDevicePixels="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0"/>
</Grid.ColumnDefinitions>
<Popup x:Name="PART_Popup" Margin="1" AllowsTransparency="true" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}"Placement="Bottom" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Grid.ColumnSpan="2">
<Microsoft_Windows_Themes:SystemDropShadowChrome x:Name="Shdw" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=MainGrid}" Color="Transparent">
<Border x:Name="DropDownBorder" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}"BorderThickness="1">
<ScrollViewer CanContentScroll="true">
<ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"KeyboardNavigation.DirectionalNavigation="Contained"/>
</ScrollViewer></Border>
</Microsoft_Windows_Themes:SystemDropShadowChrome></Popup>
<ToggleButton Style="{StaticResource ComboBoxReadonlyToggleButton}" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" Grid.ColumnSpan="2" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"/>
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" IsHitTestVisible="false" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"Content="{TemplateBinding SelectionBoxItem}" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
<Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
<Setter Property="Color" TargetName="Shdw" Value="#71000000"/></Trigger>
<Trigger Property="HasItems" Value="false">
<Setter Property="Height" TargetName="DropDownBorder" Value="95"/></Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
<Setter Property="Background" Value="#FFF4F4F4"/></Trigger>
<Trigger Property="IsGrouping" Value="true">
<Setter Property="ScrollViewer.CanContentScroll" Value="false"/></Trigger>
</ControlTemplate.Triggers>
</ControlTemplate></Setter.Value</Setter>
<Style.Triggers>
<Trigger Property="IsEditable" Value="true">
<Setter Property="BorderBrush" Value="{StaticResource TextBoxBorder}"/>
<Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"/>
<Setter Property="IsTabStop" Value="false"/>
<Setter Property="Padding" Value="3"/>
<Setter Property="Template" Value="{StaticResource ComboBoxEditableTemplate}"/>
</Trigger>
</Style.Triggers>
</Style>
</Window.Resources>
<StackPanel>
<ComboBox Style="{DynamicResource ComboBoxStyle1}">
<ComboBoxItem Content="abc"/>
<ComboBoxItem Content="abc"/>
<ComboBoxItem Content="abc"/>
</ComboBox>
</StackPanel>
</Window>

Change the Menu background when clicked in WPF

I'm using the below code to style a Menu. I have been able to change the background when the Mouse is Over the menu. But I don't know how to change its Background when it is clicked.
<Window x:Class="WpfApplication11.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<SolidColorBrush x:Key="NormalColor">White</SolidColorBrush>
<SolidColorBrush x:Key="HoverColor">LightGray</SolidColorBrush>
<SolidColorBrush x:Key="ClickedColor">DarkGray</SolidColorBrush>
<Style x:Key="{x:Type Menu}" TargetType="Menu">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Menu">
<Border x:Name="MainMenu" Background="{StaticResource NormalColor}"
BorderBrush="Black"
BorderThickness="1">
<StackPanel ClipToBounds="True" Orientation="Horizontal" IsItemsHost="True"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="MainMenu" Property="Background" Value="{StaticResource HoverColor}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Grid>
<Menu Width="30" Height="20" Margin="10, 10, 5, 5" HorizontalAlignment="Left" Background="White">
<MenuItem Header="_File">
<MenuItem Header="_New"/>
<MenuItem Header="_Open"/>
<MenuItem Header="_Close"/>
</MenuItem>
</Menu>
</Grid>
</Window>
Update:
I have tried the following code but it didn't work either.
<Trigger Property="IsFocused" Value="True">
<Setter TargetName="MainMenu" Property="Background" Value="{StaticResource ClickedColor}"/>
</Trigger

You could try using IsKeyboardFocusWithin.
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter TargetName="MainMenu" Property="Background" Value="{StaticResource ClickedColor}"/>
</Trigger>
Here is your code with the IsKeyboardFocusWithin trigger added. I changed the ClickedColor to Purple so it is easier to see. I added Name="mm" to your menu. I added a stack panel with textblocks bound to mm.IsFocused and mm.IsKeyboardFocusWithin so you can visually see what they are set to and when it changes.
<Window x:Class="WpfApplication11.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<SolidColorBrush x:Key="NormalColor">White</SolidColorBrush>
<SolidColorBrush x:Key="HoverColor">LightGray</SolidColorBrush>
<SolidColorBrush x:Key="ClickedColor">Purple</SolidColorBrush>
<Style x:Key="{x:Type Menu}" TargetType="Menu">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Menu">
<Border x:Name="MainMenu" Background="{StaticResource NormalColor}"
BorderBrush="Black"
BorderThickness="1">
<StackPanel ClipToBounds="True" Orientation="Horizontal" IsItemsHost="True"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="MainMenu" Property="Background" Value="{StaticResource HoverColor}"/>
</Trigger>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter TargetName="MainMenu" Property="Background" Value="{StaticResource ClickedColor}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Grid>
<Menu Width="30" Height="20" Margin="10, 10, 5, 5" HorizontalAlignment="Left" Background="White" VerticalAlignment="Top" Name="mm">
<MenuItem Header="_File">
<MenuItem Header="_New"/>
<MenuItem Header="_Open"/>
<MenuItem Header="_Close"/>
</MenuItem>
</Menu>
<StackPanel VerticalAlignment="Center">
<TextBlock Text="menu.IsFocused:" />
<TextBlock Text="{Binding ElementName=mm, Path=IsFocused}" />
<TextBlock Text="menu.IsKeyboardFocusWithin:" />
<TextBlock Text="{Binding ElementName=mm, Path=IsKeyboardFocusWithin}" />
</StackPanel>
</Grid>
OK, for the menu item... try this:
<Window x:Class="WpfApplication11.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<SolidColorBrush x:Key="NormalColor">White</SolidColorBrush>
<SolidColorBrush x:Key="HoverColor">LightGray</SolidColorBrush>
<SolidColorBrush x:Key="ClickedColor">Purple</SolidColorBrush>
<Style x:Key="{x:Type Menu}" TargetType="Menu">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Menu">
<Border x:Name="MainMenu" Background="{StaticResource NormalColor}"
BorderBrush="Black"
BorderThickness="1">
<StackPanel ClipToBounds="True" Orientation="Horizontal" IsItemsHost="True"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="MainMenu" Property="Background" Value="{StaticResource HoverColor}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="MenuItemControlTemplate1" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
<Grid VerticalAlignment="Center">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" Content="{TemplateBinding Icon}" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Path x:Name="GlyphPanel" Data="F1M10,1.2L4.7,9.1 4.5,9.1 0,5.2 1.3,3.5 4.3,6.1 8.3,0 10,1.2z" Fill="{TemplateBinding Foreground}" FlowDirection="LeftToRight" Margin="3" Visibility="Collapsed" VerticalAlignment="Center"/>
<ContentPresenter ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.Column="1" ContentStringFormat="{TemplateBinding HeaderStringFormat}" ContentSource="Header" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
<Popup x:Name="PART_Popup" AllowsTransparency="True" Focusable="False" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" Placement="Bottom">
<Border x:Name="SubMenuBorder" BorderBrush="#FF999999" BorderThickness="1" Background="#FFF0F0F0" Padding="2">
<ScrollViewer x:Name="SubMenuScrollViewer" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=SubMenuBorder}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<Rectangle Fill="#FFD7D7D7" HorizontalAlignment="Left" Margin="29,2,0,2" Width="1"/>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="True">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="#3D26A0DA"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#FF26A0DA"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="#FF707070"/>
<Setter Property="Fill" TargetName="GlyphPanel" Value="#FF707070"/>
</Trigger>
<Trigger Property="CanContentScroll" SourceName="SubMenuScrollViewer" Value="False">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter TargetName="templateRoot" Property="Background" Value="{StaticResource ClickedColor}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Window.Resources>
<Grid>
<Menu Width="100" Height="20" Margin="10, 10, 5, 5" HorizontalAlignment="Left" Background="White" VerticalAlignment="Top">
<MenuItem Header="_File" Name="mm" Template="{DynamicResource MenuItemControlTemplate1}">
<MenuItem Header="_New"/>
<MenuItem Header="_Open"/>
<MenuItem Header="_Close"/>
</MenuItem>
<MenuItem Header="_Edit" Template="{DynamicResource MenuItemControlTemplate1}">
<MenuItem Header="_Copy"/>
<MenuItem Header="_Cut"/>
<MenuItem Header="_Paste"/>
</MenuItem>
</Menu>
<StackPanel VerticalAlignment="Center">
<TextBlock Text="menuitem.IsFocused:" />
<TextBlock Text="{Binding ElementName=mm, Path=IsFocused}" />
<TextBlock Text="menuitem.IsKeyboardFocusWithin:" />
<TextBlock Text="{Binding ElementName=mm, Path=IsKeyboardFocusWithin}" />
</StackPanel>
</Grid>

You just need to add an event handler to the Menu.PreviewMouseDown event and change the Background there. Try this:
<Menu Width="30" Height="20" Margin="10, 10, 5, 5" HorizontalAlignment="Left"
Background="White" PreviewMouseDown="Menu_PreviewMouseDown">
<MenuItem Header="_File">
<MenuItem Header="_New"/>
<MenuItem Header="_Open"/>
<MenuItem Header="_Close"/>
</MenuItem>
</Menu>
...
private void Menu_PreviewMouseDown(object sender, MouseButtonEventArgs e)
{
Menu menu = (Menu)sender;
menu.Background = Brushes.DarkGray;
}
If you just want it to change colour while the mouse button is held down, then you could attach a handler to the MenuItem.PreviewMouseUp event to change it back to White.
UPDATE >>>
You can add a handler to the MouseLeave event to change the Backgroundback when the mouse cursor leaves the bounds of theMenu`. Try this:
<Menu Width="30" Height="20" Margin="10, 10, 5, 5" HorizontalAlignment="Left"
Background="White" PreviewMouseDown="Menu_PreviewMouseDown"
MouseLeave="Menu_MouseLeave">
<MenuItem Header="_File">
<MenuItem Header="_New"/>
<MenuItem Header="_Open"/>
<MenuItem Header="_Close"/>
</MenuItem>
</Menu>
...
private void Menu_MouseLeave(object sender, MouseEventArgs e)
{
Menu menu = (Menu)sender;
menu.Background = Brushes.White;
}

You can rework the Template for the MenuItem (not the Menu). To reproduce what I did:
Select a MenuItem in XAML
Right click it in the designer.
Select 'Edit Template'
Select 'Edit a Copy'
Edit the 'MenuItem.Highlight.Background' SolidColorBrush to a new color of your choosing. (I chose 'Red')
Remove the Key from the style for the MenuItem.
You will end up with the following resources if you start from scratch:
<Window.Resources>
<SolidColorBrush x:Key="MenuItem.Highlight.Background" Color="Red"/>
<SolidColorBrush x:Key="MenuItem.Highlight.Border" Color="#FF26A0DA"/>
<SolidColorBrush x:Key="Menu.Disabled.Foreground" Color="#FF707070"/>
<SolidColorBrush x:Key="MenuItem.Highlight.Disabled.Background" Color="#0A000000"/>
<SolidColorBrush x:Key="MenuItem.Highlight.Disabled.Border" Color="#21000000"/>
<SolidColorBrush x:Key="MenuItem.Selected.Border" Color="#FF26A0DA"/>
<SolidColorBrush x:Key="MenuItem.Selected.Background" Color="#3D26A0DA"/>
<Geometry x:Key="Checkmark">F1 M 10.0,1.2 L 4.7,9.1 L 4.5,9.1 L 0,5.2 L 1.3,3.5 L 4.3,6.1L 8.3,0 L 10.0,1.2 Z</Geometry>
<SolidColorBrush x:Key="Menu.Static.Foreground" Color="#FF212121"/>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=SubmenuItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Height="22" SnapsToDevicePixels="true">
<Grid Margin="-1">
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="22" SharedSizeGroup="MenuItemIconColumnGroup" Width="Auto"/>
<ColumnDefinition Width="13"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="30"/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
<ColumnDefinition Width="20"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Border x:Name="GlyphPanel" BorderBrush="{StaticResource MenuItem.Selected.Border}" BorderThickness="1" Background="{StaticResource MenuItem.Selected.Background}" ClipToBounds="False" HorizontalAlignment="Center" Height="22" Margin="-1,0,0,0" Visibility="Hidden" VerticalAlignment="Center" Width="22">
<Path x:Name="Glyph" Data="{StaticResource Checkmark}" Fill="{StaticResource Menu.Static.Foreground}" FlowDirection="LeftToRight" Height="11" Width="10"/>
</Border>
<ContentPresenter x:Name="menuHeaderContainer" Grid.Column="2" ContentSource="Header" HorizontalAlignment="Left" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center"/>
<TextBlock x:Name="menuGestureText" Grid.Column="4" Margin="{TemplateBinding Padding}" Opacity="0.7" Text="{TemplateBinding InputGestureText}" VerticalAlignment="Center"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="Glyph" Value="{StaticResource Menu.Disabled.Foreground}"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsHighlighted" Value="True"/>
<Condition Property="IsEnabled" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Disabled.Border}"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<SolidColorBrush x:Key="Menu.Static.Border" Color="#FF999999"/>
<SolidColorBrush x:Key="Menu.Static.Background" Color="#FFF0F0F0"/>
<SolidColorBrush x:Key="Menu.Static.Separator" Color="#FFD7D7D7"/>
<Geometry x:Key="UpArrow">M 0,4 L 3.5,0 L 7,4 Z</Geometry>
<Style x:Key="MenuScrollButton" BasedOn="{x:Null}" TargetType="{x:Type RepeatButton}">
<Setter Property="ClickMode" Value="Hover"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border x:Name="templateRoot" BorderBrush="Transparent" BorderThickness="1" Background="Transparent" SnapsToDevicePixels="true">
<ContentPresenter HorizontalAlignment="Center" Margin="6" VerticalAlignment="Center"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<MenuScrollingVisibilityConverter x:Key="MenuScrollingVisibilityConverter"/>
<Geometry x:Key="DownArrow">M 0,0 L 3.5,4 L 7,0 Z</Geometry>
<Style x:Key="{ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}" BasedOn="{x:Null}" TargetType="{x:Type ScrollViewer}">
<Setter Property="HorizontalScrollBarVisibility" Value="Hidden"/>
<Setter Property="VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollViewer}">
<Grid SnapsToDevicePixels="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Border Grid.Column="0" Grid.Row="1">
<ScrollContentPresenter CanContentScroll="{TemplateBinding CanContentScroll}" Margin="{TemplateBinding Padding}"/>
</Border>
<RepeatButton Grid.Column="0" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Command="{x:Static ScrollBar.LineUpCommand}" Focusable="false" Grid.Row="0" Style="{StaticResource MenuScrollButton}">
<RepeatButton.Visibility>
<MultiBinding ConverterParameter="0" Converter="{StaticResource MenuScrollingVisibilityConverter}" FallbackValue="Visibility.Collapsed">
<Binding Path="ComputedVerticalScrollBarVisibility" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="VerticalOffset" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ExtentHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ViewportHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
</MultiBinding>
</RepeatButton.Visibility>
<Path Data="{StaticResource UpArrow}" Fill="{StaticResource Menu.Static.Foreground}"/>
</RepeatButton>
<RepeatButton Grid.Column="0" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Command="{x:Static ScrollBar.LineDownCommand}" Focusable="false" Grid.Row="2" Style="{StaticResource MenuScrollButton}">
<RepeatButton.Visibility>
<MultiBinding ConverterParameter="100" Converter="{StaticResource MenuScrollingVisibilityConverter}" FallbackValue="Visibility.Collapsed">
<Binding Path="ComputedVerticalScrollBarVisibility" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="VerticalOffset" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ExtentHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ViewportHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
</MultiBinding>
</RepeatButton.Visibility>
<Path Data="{StaticResource DownArrow}" Fill="{StaticResource Menu.Static.Foreground}"/>
</RepeatButton>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=TopLevelHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="true">
<Grid VerticalAlignment="Center">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Path x:Name="GlyphPanel" Data="{StaticResource Checkmark}" Fill="{TemplateBinding Foreground}" FlowDirection="LeftToRight" Margin="3" Visibility="Collapsed" VerticalAlignment="Center"/>
<ContentPresenter Grid.Column="1" ContentSource="Header" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
<Popup x:Name="PART_Popup" AllowsTransparency="true" Focusable="false" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" Placement="Bottom" PlacementTarget="{Binding ElementName=templateRoot}">
<Border x:Name="SubMenuBorder" BorderBrush="{StaticResource Menu.Static.Border}" BorderThickness="1" Background="{StaticResource Menu.Static.Background}" Padding="2">
<ScrollViewer x:Name="SubMenuScrollViewer" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=SubMenuBorder}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<Rectangle Fill="{StaticResource Menu.Static.Separator}" HorizontalAlignment="Left" Margin="29,2,0,2" Width="1"/>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="true" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="true">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="GlyphPanel" Value="{StaticResource Menu.Disabled.Foreground}"/>
</Trigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="SubMenuScrollViewer" Value="false">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=TopLevelItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="true">
<Grid VerticalAlignment="Center">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Path x:Name="GlyphPanel" Data="{StaticResource Checkmark}" Fill="{StaticResource Menu.Static.Foreground}" FlowDirection="LeftToRight" Margin="3" Visibility="Collapsed" VerticalAlignment="Center"/>
<ContentPresenter Grid.Column="1" ContentSource="Header" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="GlyphPanel" Value="{StaticResource Menu.Disabled.Foreground}"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsHighlighted" Value="True"/>
<Condition Property="IsEnabled" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Disabled.Border}"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Geometry x:Key="RightArrow">M 0,0 L 4,3.5 L 0,7 Z</Geometry>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=SubmenuHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Height="22" SnapsToDevicePixels="true">
<Grid Margin="-1">
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="22" SharedSizeGroup="MenuItemIconColumnGroup" Width="Auto"/>
<ColumnDefinition Width="13"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="30"/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
<ColumnDefinition Width="20"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Border x:Name="GlyphPanel" BorderBrush="{StaticResource MenuItem.Highlight.Border}" BorderThickness="1" Background="{StaticResource MenuItem.Highlight.Background}" Height="22" Margin="-1,0,0,0" Visibility="Hidden" VerticalAlignment="Center" Width="22">
<Path x:Name="Glyph" Data="{DynamicResource Checkmark}" Fill="{StaticResource Menu.Static.Foreground}" FlowDirection="LeftToRight" Height="11" Width="9"/>
</Border>
<ContentPresenter Grid.Column="2" ContentSource="Header" HorizontalAlignment="Left" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center"/>
<TextBlock Grid.Column="4" Margin="{TemplateBinding Padding}" Opacity="0.7" Text="{TemplateBinding InputGestureText}" VerticalAlignment="Center"/>
<Path x:Name="RightArrow" Grid.Column="5" Data="{StaticResource RightArrow}" Fill="{StaticResource Menu.Static.Foreground}" HorizontalAlignment="Left" Margin="10,0,0,0" VerticalAlignment="Center"/>
<Popup x:Name="PART_Popup" AllowsTransparency="true" Focusable="false" HorizontalOffset="-2" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" Placement="Right" VerticalOffset="-3">
<Border x:Name="SubMenuBorder" BorderBrush="{StaticResource Menu.Static.Border}" BorderThickness="1" Background="{StaticResource Menu.Static.Background}" Padding="2">
<ScrollViewer x:Name="SubMenuScrollViewer" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=SubMenuBorder}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<Rectangle Fill="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}" HorizontalAlignment="Left" Margin="29,2,0,2" Width="1"/>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="true" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="true">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="Transparent"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="Glyph" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="RightArrow" Value="{StaticResource Menu.Disabled.Foreground}"/>
</Trigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="SubMenuScrollViewer" Value="false">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Style TargetType="{x:Type MenuItem}">
<Setter Property="HorizontalContentAlignment" Value="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderBrush" Value="Transparent"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="ScrollViewer.PanningMode" Value="Both"/>
<Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
<Setter Property="Template" Value="{DynamicResource {ComponentResourceKey ResourceId=SubmenuItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}}"/>
<Style.Triggers>
<Trigger Property="Role" Value="TopLevelHeader">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderBrush" Value="Transparent"/>
<Setter Property="Foreground" Value="{StaticResource Menu.Static.Foreground}"/>
<Setter Property="Template" Value="{DynamicResource {ComponentResourceKey ResourceId=TopLevelHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}}"/>
<Setter Property="Padding" Value="6,0"/>
</Trigger>
<Trigger Property="Role" Value="TopLevelItem">
<Setter Property="Background" Value="{StaticResource Menu.Static.Background}"/>
<Setter Property="BorderBrush" Value="{StaticResource Menu.Static.Border}"/>
<Setter Property="Foreground" Value="{StaticResource Menu.Static.Foreground}"/>
<Setter Property="Template" Value="{DynamicResource {ComponentResourceKey ResourceId=TopLevelItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}}"/>
<Setter Property="Padding" Value="6,0"/>
</Trigger>
<Trigger Property="Role" Value="SubmenuHeader">
<Setter Property="Template" Value="{DynamicResource {ComponentResourceKey ResourceId=SubmenuHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}}"/>
</Trigger>
</Style.Triggers>
</Style>
</Window.Resources>
This looks daunting, but you are basically telling the MenuItem to use another Template other than the one defined in the theme. Personally, I avoid doing such things, as it breaks the themes across the various Windows versions. By doing this, the Menu will look the same on XP, Vista & 8.
You will likely be able to optimize this quite a bit. But I at least recommend you put it in a separate ResourceDictionary.

Enable highlighting of MenuItem outside of ContentPresenter

I've been at it for a while and it's driving me crazy. Here's my problem... I have a Menu and it uses the default template in which I've altered colors upon highlighting, default state, etc. So, there's no point posting it here -- it's virtually exact same thing (and it's a lot of template code).
What my problem is the spacing between the MenuItems created by the template's Marginproperty. It is template-binded with the padding property and produces a (2, 3, 2, 3) margin all around the ContentPresenter of the template.
I want to keep that property and need it to separate the items; however, mousing over only activates highlighted state when the mouse is over the ContentPresenter and nothing else. So, the "button" feel won't actuate until I mouse over the words themselves. ContentPresenter does not have padding, so I'm not sure how to deal with it. I tried editing the triggers that came with the template to work on other parts, but it doesn't work (they don't fire off).
The triggers that come with the template only work with the ContentPresenter. I tried wrapping it in ContentControl to see if I can gain Padding, but the binding for Padding doesn't work, only for margin... not sure why (it shows up in the Properties, but the addition of padding is not visible):
<ContentControl SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
Padding="{TemplateBinding Padding}">
<ContentPresenter ContentSource="Header" RecognizesAccessKey="True"/>
</ContentControl>
I also tried replacing ContentPresenter with ContentControl altogether, but once again suffered from Padding not binding properly and manual Padding didn't do anything either. Something is overwriting it, perhaps.
Any idea how I can actuate triggers in the empty space between those MenuItems? The way I've been dealing with it is by adding whitespace into my MenuItem headers... and that's just absurd.
Here's my template:
<LinearGradientBrush x:Key="MenuItemSelectionFill" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#34C5EBFF" Offset="0"/>
<GradientStop Color="#3481D8FF" Offset="1"/>
</LinearGradientBrush>
<Geometry x:Key="Checkmark">M 0,5.1 L 1.7,5.2 L 3.4,7.1 L 8,0.4 L 9.2,0 L 3.3,10.8 Z</Geometry>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=SubmenuItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Grid SnapsToDevicePixels="true" Background="#FFEFF6FB" HorizontalAlignment="Stretch">
<Rectangle x:Name="Bg"/>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="24" SharedSizeGroup="MenuItemIconColumnGroup" Width="Auto"/>
<ColumnDefinition Width="4"/>
<ColumnDefinition/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
<ColumnDefinition Width="17"/>
</Grid.ColumnDefinitions>
<!--<ContentPresenter x:Name="Icon" VerticalAlignment="Center" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" Margin="1,12" ContentSource="Icon" Height="0"/>
<Border x:Name="GlyphPanel" Width="Auto" Visibility="Hidden" Margin="1" Height="Auto" CornerRadius="3" Background="#FFEFF6FB" BorderThickness="1" BorderBrush="#FFEFF6FB">
<Path x:Name="Glyph" Width="9" Height="11" FlowDirection="LeftToRight" Fill="#0C12A1" Data="{StaticResource Checkmark}"/>
</Border>-->
<ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" RecognizesAccessKey="True" Margin="{TemplateBinding Padding}" ContentSource="Header" Grid.Column="2"/>
<TextBlock Grid.Column="3" Margin="{TemplateBinding Padding}" Text="{TemplateBinding InputGestureText}"/>
</Grid>
</Grid>
<ControlTemplate.Triggers>
<!--<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>-->
<!--<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>-->
<Trigger Property="IsHighlighted" Value="true">
<Setter Property="Fill" TargetName="Bg" Value="#FFC0E5FF"/>
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="#FF9A9A9A"/>
<!--<Setter Property="Background" TargetName="GlyphPanel" Value="#EEE9E9"/>
<Setter Property="BorderBrush" TargetName="GlyphPanel" Value="#DBD6D6"/>
<Setter Property="Fill" TargetName="Glyph" Value="#848589"/>-->
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<LinearGradientBrush x:Key="MenuItemPressedFill" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#28717070" Offset="0"/>
<GradientStop Color="#50717070" Offset="0.75"/>
<GradientStop Color="#90717070" Offset="1"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="SubMenuBackgroundBrush" Color="#FFEFF6FB"/>
<Geometry x:Key="UpArrow">M 0,4 L 3.5,0 L 7,4 Z</Geometry>
<Style x:Key="MenuScrollButton" BasedOn="{x:Null}" TargetType="{x:Type RepeatButton}">
<Setter Property="ClickMode" Value="Hover"/>
<Setter Property="MinWidth" Value="0"/>
<Setter Property="MinHeight" Value="0"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<DockPanel Background="Transparent" SnapsToDevicePixels="true">
<Rectangle x:Name="R1" Width="1" Fill="Transparent" DockPanel.Dock="Right"/>
<Rectangle x:Name="B1" Height="1" Fill="Transparent" DockPanel.Dock="Bottom"/>
<Rectangle x:Name="L1" Width="1" Fill="Transparent" DockPanel.Dock="Left"/>
<Rectangle x:Name="T1" Height="1" Fill="Transparent" DockPanel.Dock="Top"/>
<ContentPresenter x:Name="ContentContainer" VerticalAlignment="Center" Margin="2,2,2,2" HorizontalAlignment="Center"/>
</DockPanel>
<ControlTemplate.Triggers>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Fill" TargetName="R1" Value="{DynamicResource {x:Static SystemColors.ControlLightLightBrushKey}}"/>
<Setter Property="Fill" TargetName="B1" Value="{DynamicResource {x:Static SystemColors.ControlLightLightBrushKey}}"/>
<Setter Property="Fill" TargetName="L1" Value="{DynamicResource {x:Static SystemColors.ControlDarkDarkBrushKey}}"/>
<Setter Property="Fill" TargetName="T1" Value="{DynamicResource {x:Static SystemColors.ControlDarkDarkBrushKey}}"/>
<Setter Property="Margin" TargetName="ContentContainer" Value="3,3,1,1"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<MenuScrollingVisibilityConverter x:Key="MenuScrollingVisibilityConverter"/>
<Geometry x:Key="DownArrow">M 0,0 L 3.5,4 L 7,0 Z</Geometry>
<Style x:Key="{ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}" BasedOn="{x:Null}" TargetType="{x:Type ScrollViewer}">
<Setter Property="HorizontalScrollBarVisibility" Value="Hidden"/>
<Setter Property="VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollViewer}">
<Grid SnapsToDevicePixels="true">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Border Grid.Column="0" Grid.Row="1">
<ScrollContentPresenter CanContentScroll="{TemplateBinding CanContentScroll}" Margin="{TemplateBinding Padding}"/>
</Border>
<RepeatButton Style="{StaticResource MenuScrollButton}" Grid.Row="0" Focusable="false" Command="{x:Static ScrollBar.LineUpCommand}" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Grid.Column="0">
<RepeatButton.Visibility>
<MultiBinding FallbackValue="Visibility.Collapsed" Converter="{StaticResource MenuScrollingVisibilityConverter}" ConverterParameter="0">
<Binding Path="ComputedVerticalScrollBarVisibility" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="VerticalOffset" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ExtentHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ViewportHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
</MultiBinding>
</RepeatButton.Visibility>
<Path Data="{StaticResource UpArrow}" Fill="{DynamicResource {x:Static SystemColors.MenuTextBrushKey}}"/>
</RepeatButton>
<RepeatButton Style="{StaticResource MenuScrollButton}" Grid.Row="2" Focusable="false" Command="{x:Static ScrollBar.LineDownCommand}" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Grid.Column="0">
<RepeatButton.Visibility>
<MultiBinding FallbackValue="Visibility.Collapsed" Converter="{StaticResource MenuScrollingVisibilityConverter}" ConverterParameter="100">
<Binding Path="ComputedVerticalScrollBarVisibility" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="VerticalOffset" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ExtentHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ViewportHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
</MultiBinding>
</RepeatButton.Visibility>
<Path Data="{StaticResource DownArrow}" Fill="{DynamicResource {x:Static SystemColors.MenuTextBrushKey}}"/>
</RepeatButton>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=TopLevelHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Grid SnapsToDevicePixels="true">
<Rectangle x:Name="OuterBorder"/>
<Rectangle x:Name="Bg"/>
<DockPanel>
<!--<ContentPresenter x:Name="Icon" ContentSource="Icon" Margin="0" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center"/>
<Path x:Name="GlyphPanel" Data="{StaticResource Checkmark}" Fill="{TemplateBinding Foreground}" FlowDirection="LeftToRight" Margin="0" Visibility="Collapsed" VerticalAlignment="Center"/>-->
<ContentPresenter ContentSource="Header" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" Margin="0,1"/>
</DockPanel>
<Popup x:Name="PART_Popup" AllowsTransparency="true" Focusable="false" HorizontalOffset="0" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" Placement="Bottom" VerticalOffset="0">
<Themes:SystemDropShadowChrome x:Name="Shdw" Color="Transparent">
<Border x:Name="SubMenuBorder" Background="{StaticResource SubMenuBackgroundBrush}">
<ScrollViewer x:Name="SubMenuScrollViewer" Margin="1,0" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{StaticResource SubMenuBackgroundBrush}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="true" Margin="2" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Themes:SystemDropShadowChrome>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="true">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<!--<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>-->
<Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
<Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
<Setter Property="Color" TargetName="Shdw" Value="#71000000"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="true">
<Setter Property="Fill" TargetName="Bg" Value="#FFEFF6FB"/>
<Setter Property="Foreground" Value="Black"/>
<!--<Setter Property="Stroke" TargetName="Bg" Value="#90717070"/>-->
<!--<Setter Property="Stroke" TargetName="OuterBorder" Value="#50FFFFFF"/>-->
<!--<Setter Property="Stroke" TargetName="InnerBorder" Value="#50FFFFFF"/>-->
</Trigger>
<Trigger Property="IsKeyboardFocused" Value="true">
<!--<Setter Property="Stroke" TargetName="Bg" Value="#E0717070"/>-->
<Setter Property="Fill" TargetName="Bg" Value="#FFEFF6FB"/>
<Setter Property="Foreground" Value="Black"/>
<!--<Setter Property="Stroke" TargetName="InnerBorder" Value="#50747272"/>-->
</Trigger>
<Trigger Property="IsSubmenuOpen" Value="true">
<Setter Property="Fill" TargetName="Bg" Value="#FFEFF6FB"/>
<Setter Property="Foreground" Value="Black"/>
<Setter Property="Fill" TargetName="OuterBorder" Value="{x:Null}"/>
<Setter Property="Stroke" TargetName="OuterBorder" Value="{x:Null}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="#FF9A9A9A"/>
<!--<Setter Property="Fill" TargetName="GlyphPanel" Value="#848589"/>-->
</Trigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="SubMenuScrollViewer" Value="false">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=TopLevelItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Grid SnapsToDevicePixels="true">
<Rectangle x:Name="Bg"/>
<DockPanel>
<ContentPresenter x:Name="Icon" ContentSource="Icon" Margin="4,0,6,0" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center"/>
<Path x:Name="GlyphPanel" Data="{StaticResource Checkmark}" Fill="{TemplateBinding Foreground}" FlowDirection="LeftToRight" Margin="7,0,0,0" Visibility="Collapsed" VerticalAlignment="Center"/>
<ContentPresenter ContentSource="Header" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</DockPanel>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="true">
<Setter Property="Fill" TargetName="Bg" Value="#FFEFF6FB"/>
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="IsKeyboardFocused" Value="true">
<Setter Property="Fill" TargetName="Bg" Value="#FFEFF6FB"/>
<Setter Property="Foreground" Value="Black"/>
<!--<Setter Property="Fill" TargetName="Bg" Value="{StaticResource MenuItemPressedFill}"/>-->
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="#FF9A9A9A"/>
<Setter Property="Fill" TargetName="GlyphPanel" Value="#848589"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Geometry x:Key="RightArrow">M 0,0 L 4,3.5 L 0,7 Z</Geometry>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=SubmenuHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Grid SnapsToDevicePixels="true">
<Rectangle x:Name="Bg"/>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="24" SharedSizeGroup="MenuItemIconColumnGroup" Width="Auto"/>
<ColumnDefinition Width="4"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="37"/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
<ColumnDefinition Width="17"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" Margin="1" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center"/>
<Border x:Name="GlyphPanel" BorderBrush="#CDD3E6" BorderThickness="1" Background="#E6EFF4" CornerRadius="3" Height="22" Margin="1" Visibility="Hidden" Width="22">
<Path x:Name="Glyph" Data="{StaticResource Checkmark}" Fill="#0C12A1" FlowDirection="LeftToRight" Height="11" Width="9"/>
</Border>
<ContentPresenter Grid.Column="2" ContentSource="Header" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
<TextBlock Grid.Column="4" Margin="{TemplateBinding Padding}" Text="{TemplateBinding InputGestureText}" Visibility="Collapsed"/>
<Path Grid.Column="5" Data="{StaticResource RightArrow}" Fill="{TemplateBinding Foreground}" Margin="4,0,0,0" VerticalAlignment="Center"/>
</Grid>
<Popup x:Name="PART_Popup" AllowsTransparency="true" Focusable="false" HorizontalOffset="-2" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" Placement="Right" VerticalOffset="-3">
<Themes:SystemDropShadowChrome x:Name="Shdw" Color="Transparent">
<Border x:Name="SubMenuBorder">
<ScrollViewer x:Name="SubMenuScrollViewer" Margin="1,0" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{StaticResource SubMenuBackgroundBrush}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="true" Margin="2" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Themes:SystemDropShadowChrome>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="true">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="true">
<Setter Property="Fill" TargetName="Bg" Value="#FFEFF6FB"/>
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
<Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
<Setter Property="Color" TargetName="Shdw" Value="#71000000"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="#FF9A9A9A"/>
<Setter Property="Background" TargetName="GlyphPanel" Value="#EEE9E9"/>
<Setter Property="BorderBrush" TargetName="GlyphPanel" Value="#DBD6D6"/>
<Setter Property="Fill" TargetName="Glyph" Value="#848589"/>
</Trigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="SubMenuScrollViewer" Value="false">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>

The problem is in the MenuItem template with the ResourceID "TopLevelItemTemplateKey".
Note that in this template no background is set for the transparent areas of the menu item. Technically speaking, the background in these areas is not transparent, it is null - not setting a background will for many UI elements let the mouse hit test fail in regions without content (hit-testing is not done pixel-by-pixel but rather by specific UIElement.InputHitTest method implementations).
Setting the background explicitly to "Transparent" on the outermost UI element in your template will fix this issue:
<ControlTemplate
x:Key="{ComponentResourceKey ResourceId=TopLevelItemTemplateKey,
TypeInTargetAssembly={x:Type MenuItem}}"
TargetType="{x:Type MenuItem}"
>
<!-- ### FIX: SETTING THE GRID BACKGROUND TO "TRANSPARENT" ### -->
<Grid Background="Transparent" SnapsToDevicePixels="true">
<Rectangle x:Name="Bg"/>
...
</Grid>
</ControlTemplate>
(If you want to know more about hit testing in WPF, read here in the MSDN documentation.)

WPF Change Background color of a Combobox

In my WPF app I just want to change the background color of the Combo box. I don't mean the dropdown, I want is just whatever item is selected a background is set.
Like setting the background of a button - when the control is displayed on screen it should have LightYellow background. That's it.
I searched a lot on net, but everywhere could find solutions for drop down background colors. I tried applying SolidColorBrush and Style.Triggers to the TextBlock of Combobox, but no success as wanted. By adding SolidColorBrush lines, I got my dropdown background set, but that's not what I am looking for. My code is :
<ComboBox ItemsSource="{Binding MtrCm}" SelectedValue="{Binding WellboreDiameter_Unit, Mode=TwoWay}" Grid.Row="1" Height="23" HorizontalAlignment="Right" Margin="0,26,249,0" x:Name="cboWellDiameter" VerticalAlignment="Top" Width="120" Background="LightYellow" >
<ComboBox.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.WindowBrushKey}" Color="Yellow" />
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightTextBrushKey}" Color="Yellow" />
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Yellow" />
<Style TargetType="TextBlock">
<Style.Triggers>
<DataTrigger Binding="{Binding IsSelected, RelativeSource={RelativeSource AncestorType=ComboBoxItem}}" Value="True">
<Setter Property="Background" Value="Red" />
</DataTrigger>
</Style.Triggers>
</Style>
</ComboBox.Resources>
</ComboBox>
Can anyone help me set he background of the desired component that am looking for.
Thanks

Try this
<Window.Resources> //Put this resourse n Window.Resources or UserControl.Resources
<LinearGradientBrush x:Key="NormalBrush" StartPoint="0,0" EndPoint="0,1">
<GradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="#FFDC3939" Offset="0.0"/>
<GradientStop Color="#FFE80E0E" Offset="1.0"/>
</GradientStopCollection>
</GradientBrush.GradientStops>
</LinearGradientBrush>
<SolidColorBrush x:Key="WindowBackgroundBrush" Color="#FFFBE618" />
<ControlTemplate x:Key="ComboBoxToggleButton" TargetType="ToggleButton">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="20" />
</Grid.ColumnDefinitions>
<Border x:Name="Border" Grid.ColumnSpan="2" CornerRadius="2"
Background="{StaticResource NormalBrush}"
BorderThickness="1" />
<Border
Grid.Column="0"
CornerRadius="2,0,0,2"
Margin="1"
Background="{StaticResource WindowBackgroundBrush}"
BorderThickness="0,0,1,0" />
<Path
x:Name="Arrow"
Grid.Column="1"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Data="M 0 0 L 4 4 L 8 0 Z"/>
</Grid>
</ControlTemplate>
<ControlTemplate x:Key="ComboBoxTextBox" TargetType="TextBox">
<Border x:Name="PART_ContentHost" Focusable="False" Background="{TemplateBinding Background}" />
</ControlTemplate>
<Style x:Key="{x:Type ComboBox}" TargetType="ComboBox">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<ToggleButton
Name="ToggleButton"
Template="{StaticResource ComboBoxToggleButton}"
Grid.Column="2"
Focusable="false"
IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
ClickMode="Press">
</ToggleButton>
<ContentPresenter
Name="ContentSite"
IsHitTestVisible="False"
Margin="3,3,23,3"
VerticalAlignment="Center"
HorizontalAlignment="Left" />
<TextBox x:Name="PART_EditableTextBox"
Style="{x:Null}"
Template="{StaticResource ComboBoxTextBox}"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Margin="3,3,23,3"
Focusable="True"
Background="Transparent"
Visibility="Hidden"
IsReadOnly="{TemplateBinding IsReadOnly}"/>
<Popup
Name="Popup"
Placement="Bottom"
IsOpen="{TemplateBinding IsDropDownOpen}"
AllowsTransparency="True"
Focusable="False"
PopupAnimation="Slide">
<Grid
Name="DropDown"
SnapsToDevicePixels="True"
MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}">
<Border
x:Name="DropDownBorder"
Background="{StaticResource WindowBackgroundBrush}"
BorderThickness="1"/>
<ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True">
<StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
</Style.Triggers>
</Style>
</Window.Resources>
<Grid>
<ComboBox HorizontalAlignment="Left" Margin="256,57,0,0" VerticalAlignment="Top" Width="120">
</ComboBox>
</Grid>
Here is the complete style that you can change : http://msdn.microsoft.com/en-us/library/ms752094%28v=VS.85%29.aspx

While I like the accepted answer the problem I have with it is the text box presenter was completely missed and thus you can select and stylize things but they will never be presented to the end user. I think the easiest way to achieve a combo-box that shows data and then when chosen displays it would be this:
<Window http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:Test" x:Class="MainWindow"
mc:Ignorable="d" Title="MainWindow" Height="100" Width="200">
<Window.Resources>
<Style x:Key="ComboBoxTest2" TargetType="{x:Type ComboBox}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<ToggleButton Grid.Column="2" Focusable="false" IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}" >
<ToggleButton.Template>
<ControlTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="5*" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Border x:Name="Border" Grid.ColumnSpan="2" CornerRadius="5" Background="Beige" BorderBrush="Black" BorderThickness="1" />
<Border Grid.Column="0" CornerRadius="5,0,0,5" Margin="1" Background="AliceBlue" BorderBrush="Black" BorderThickness="0,0,1,0" />
<Path x:Name="Arrow" Grid.Column="1" Fill="Orange" HorizontalAlignment="Center" VerticalAlignment="Center" Data="M 0 0 L 4 4 L 8 0 Z"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="ToggleButton.IsMouseOver" Value="true">
<Setter TargetName="Border" Property="Background" Value="Green" />
</Trigger>
<Trigger Property="ToggleButton.IsChecked" Value="true">
<Setter TargetName="Border" Property="Background" Value="Green" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</ToggleButton.Template>
</ToggleButton>
<ContentPresenter Name="ContentSite" IsHitTestVisible="False" Content="{TemplateBinding SelectionBoxItem}" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Margin="3" />
<TextBox x:Name="PART_EditableTextBox" Visibility="Hidden" IsReadOnly="{TemplateBinding IsReadOnly}"/>
<Popup Name="Popup" Placement="Bottom" IsOpen="{TemplateBinding IsDropDownOpen}" AllowsTransparency="True" Focusable="False" PopupAnimation="Slide">
<Grid Name="DropDown" SnapsToDevicePixels="True" MinWidth="{TemplateBinding ActualWidth}" MaxHeight="{TemplateBinding MaxDropDownHeight}">
<Border x:Name="DropDownBorder" Background="Blue" />
<ScrollViewer SnapsToDevicePixels="True">
<StackPanel IsItemsHost="True" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
</Style.Triggers>
</Style>
</Window.Resources>
<StackPanel Margin="10">
<ComboBox VerticalAlignment="Top" Width="120" Style="{StaticResource ComboBoxTest2}">
<ComboBoxItem>Item1</ComboBoxItem>
<ComboBoxItem>Item2</ComboBoxItem>
</ComboBox>
</StackPanel>
</Window>
The key parts of the presentation needed to be done is the template 'ToggleButton.Template' for the initial display. I chose to ignore making more brushes and links to more templates and just do it all inline so others could play with it as needed. I also chose what I thought were easily Identifiable colors to just jump to for reference it should look like below when it works:

If you are using VS 2012 or VS 2013 in the Document Outline window while viewing your xaml get the control template of a combobox.
Find the style x:Key="ComboBoxReadonlyToggleButton" and update the RenderMouseOver = "False" and RenderPressed="False" on the Themes:ButtonChrome and also comment out the trigger that affects IsEnabled.
Below is the one on I edited on my box to illustrate what I mean.
<Style x:Key="ComboBoxReadonlyToggleButton" TargetType="{x:Type ToggleButton}">
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="IsTabStop" Value="false"/>
<Setter Property="Focusable" Value="false"/>
<Setter Property="ClickMode" Value="Press"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Themes:ButtonChrome x:Name="Chrome" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" RenderMouseOver="False" RenderPressed="False" SnapsToDevicePixels="true">
<Grid HorizontalAlignment="Right" Width="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}">
<Path x:Name="Arrow" Data="{StaticResource DownArrowGeometry}" Fill="Black" HorizontalAlignment="Center" Margin="3,1,0,0" VerticalAlignment="Center"/>
</Grid>
</Themes:ButtonChrome>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="true">
<Setter Property="RenderPressed" TargetName="Chrome" Value="False"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<!--<Setter Property="Fill" TargetName="Arrow" Value="#AFAFAF"/>-->
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

It is possible to put a TextBlock onto the combobox, but the TextBlock should be smaller than the ComboBox so that the drop down button is not hidden. In the TextBlock the desired background color can easily be set. When the selected item in the ComboBox changes, the text block has to be updated.
XAML code:
<ComboBox x:Name="TestComboBox" HorizontalAlignment="Left" VerticalAlignment="Top" Width="145"
Margin="35, 5, 0, 0" SelectionChanged="TestComboBox_SelectionChanged"/>
<TextBlock x:Name="TestTextBlock" Text="" HorizontalAlignment="Left" VerticalAlignment="Top"
Margin="36, 6, 0, 0" Padding="4,1" Width="126" Height="20">
<TextBlock.Background>
<SolidColorBrush Color="Yellow"/>
</TextBlock.Background>
</TextBlock>
C# code:
private void TestComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
ComboBoxItem cbi = (ComboBoxItem)TestComboBox.SelectedItem;
if (cbi != null)
{
TestTextBlock.Text = cbi.Content.ToString();
}
else TestTextBlock.Text = "";
}

On windows 8 or 10 this is proper WPF way without redesign the Combobox and 100% customizable.
Just copy paste and test! :)
Click here to see how look the ComboBox
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:Test_Combo_Custom"
xmlns:Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero2" x:Class="Test_Combo_Custom.MainWindow"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Window.Resources>
<Style x:Key="FocusVisual">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle Margin="2" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<SolidColorBrush x:Key="ComboBox.Static.Background" Color="Yellow"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Background" Color="#FFFF0C00"/>
<SolidColorBrush x:Key="ComboBox.Static.Border" Color="#FFACACAC"/>
<SolidColorBrush x:Key="ComboBox.Static.Editable.Background" Color="#FFFB0F0F"/>
<SolidColorBrush x:Key="ComboBox.Static.Editable.Border" Color="#FFABADB3"/>
<SolidColorBrush x:Key="ComboBox.Static.Editable.Button.Background" Color="Transparent"/>
<SolidColorBrush x:Key="ComboBox.Static.Editable.Button.Border" Color="Transparent"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Glyph" Color="#FF000000"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Border" Color="#FF7EB4EA"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Editable.Background" Color="#FF88E00A"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Editable.Border" Color="#FF7EB4EA"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Editable.Button.Background" Color="#FF7EB4EA"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Editable.Button.Border" Color="#FF7EB4EA"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Glyph" Color="#FF000000"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Background" Color="#FF000000"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Border" Color="#FF569DE5"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Editable.Background" Color="#FF00FFC5"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Editable.Border" Color="#FF569DE5"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Editable.Button.Background" Color="#FF569DE5"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Editable.Button.Border" Color="#FF569DE5"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Glyph" Color="#FFBFBFBF"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Background" Color="#FF101AEC"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Border" Color="#FFD9D9D9"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Editable.Background" Color="#FFC91355"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Editable.Border" Color="#FFBFBFBF"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Editable.Button.Background" Color="Transparent"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Editable.Button.Border" Color="Transparent"/>
<SolidColorBrush x:Key="ComboBox.Static.Glyph" Color="#FF606060"/>
<Style x:Key="ComboBoxToggleButton2" TargetType="{x:Type ToggleButton}">
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="IsTabStop" Value="false"/>
<Setter Property="Focusable" Value="false"/>
<Setter Property="ClickMode" Value="Press"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border x:Name="templateRoot" BorderBrush="{StaticResource ComboBox.Static.Border}" BorderThickness="{TemplateBinding BorderThickness}" Background="{StaticResource ComboBox.Static.Background}" SnapsToDevicePixels="true">
<Border x:Name="splitBorder" BorderBrush="Transparent" BorderThickness="1" HorizontalAlignment="Right" Margin="0" SnapsToDevicePixels="true" Width="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}">
<Path x:Name="arrow" Data="F1 M 0,0 L 2.667,2.66665 L 5.3334,0 L 5.3334,-1.78168 L 2.6667,0.88501 L0,-1.78168 L0,0 Z" Fill="{StaticResource ComboBox.Static.Glyph}" HorizontalAlignment="Center" Margin="0" VerticalAlignment="Center"/>
</Border>
</Border>
<ControlTemplate.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource AncestorType={x:Type ComboBox}}}" Value="true"/>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="true"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource ComboBox.Static.Editable.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource ComboBox.Static.Editable.Border}"/>
<Setter Property="Background" TargetName="splitBorder" Value="{StaticResource ComboBox.Static.Editable.Button.Background}"/>
<Setter Property="BorderBrush" TargetName="splitBorder" Value="{StaticResource ComboBox.Static.Editable.Button.Border}"/>
</MultiDataTrigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Fill" TargetName="arrow" Value="{StaticResource ComboBox.MouseOver.Glyph}"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource AncestorType={x:Type ComboBox}}}" Value="false"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource ComboBox.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource ComboBox.MouseOver.Border}"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource AncestorType={x:Type ComboBox}}}" Value="true"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource ComboBox.MouseOver.Editable.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource ComboBox.MouseOver.Editable.Border}"/>
<Setter Property="Background" TargetName="splitBorder" Value="{StaticResource ComboBox.MouseOver.Editable.Button.Background}"/>
<Setter Property="BorderBrush" TargetName="splitBorder" Value="{StaticResource ComboBox.MouseOver.Editable.Button.Border}"/>
</MultiDataTrigger>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Fill" TargetName="arrow" Value="{StaticResource ComboBox.Pressed.Glyph}"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource AncestorType={x:Type ComboBox}}}" Value="false"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource ComboBox.Pressed.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource ComboBox.Pressed.Border}"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource AncestorType={x:Type ComboBox}}}" Value="true"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource ComboBox.Pressed.Editable.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource ComboBox.Pressed.Editable.Border}"/>
<Setter Property="Background" TargetName="splitBorder" Value="{StaticResource ComboBox.Pressed.Editable.Button.Background}"/>
<Setter Property="BorderBrush" TargetName="splitBorder" Value="{StaticResource ComboBox.Pressed.Editable.Button.Border}"/>
</MultiDataTrigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Fill" TargetName="arrow" Value="{StaticResource ComboBox.Disabled.Glyph}"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource AncestorType={x:Type ComboBox}}}" Value="false"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource ComboBox.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource ComboBox.Disabled.Border}"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource AncestorType={x:Type ComboBox}}}" Value="true"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource ComboBox.Disabled.Editable.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource ComboBox.Disabled.Editable.Border}"/>
<Setter Property="Background" TargetName="splitBorder" Value="{StaticResource ComboBox.Disabled.Editable.Button.Background}"/>
<Setter Property="BorderBrush" TargetName="splitBorder" Value="{StaticResource ComboBox.Disabled.Editable.Button.Border}"/>
</MultiDataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="ComboBoxTemplate" TargetType="{x:Type ComboBox}">
<Grid x:Name="templateRoot" SnapsToDevicePixels="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0"/>
</Grid.ColumnDefinitions>
<Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
<Themes:SystemDropShadowChrome x:Name="shadow" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=templateRoot}">
<Border x:Name="dropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
<ScrollViewer x:Name="DropDownScrollViewer">
<Grid x:Name="grid" RenderOptions.ClearTypeHint="Enabled">
<Canvas x:Name="canvas" HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="opaqueRect" Fill="{Binding Background, ElementName=dropDownBorder}" Height="{Binding ActualHeight, ElementName=dropDownBorder}" Width="{Binding ActualWidth, ElementName=dropDownBorder}"/>
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Grid>
</ScrollViewer>
</Border>
</Themes:SystemDropShadowChrome>
</Popup>
<ToggleButton x:Name="toggleButton" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ComboBoxToggleButton2}"/>
<ContentPresenter x:Name="contentPresenter" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Content="{TemplateBinding SelectionBoxItem}" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" IsHitTestVisible="false" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
<Setter Property="Margin" TargetName="shadow" Value="0,0,5,5"/>
<Setter Property="Color" TargetName="shadow" Value="#71000000"/>
</Trigger>
<Trigger Property="HasItems" Value="false">
<Setter Property="Height" TargetName="dropDownBorder" Value="95"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsGrouping" Value="true"/>
<Condition Property="VirtualizingPanel.IsVirtualizingWhenGrouping" Value="false"/>
</MultiTrigger.Conditions>
<Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
</MultiTrigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="DropDownScrollViewer" Value="false">
<Setter Property="Canvas.Top" TargetName="opaqueRect" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="opaqueRect" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<SolidColorBrush x:Key="TextBox.Static.Background" Color="#FFFFFFFF"/>
<Style x:Key="ComboBoxEditableTextBox" TargetType="{x:Type TextBox}">
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="AllowDrop" Value="true"/>
<Setter Property="MinWidth" Value="0"/>
<Setter Property="MinHeight" Value="0"/>
<Setter Property="FocusVisualStyle" Value="{x:Null}"/>
<Setter Property="ScrollViewer.PanningMode" Value="VerticalFirst"/>
<Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<ScrollViewer x:Name="PART_ContentHost" Background="Transparent" Focusable="false" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="ComboBoxEditableTemplate" TargetType="{x:Type ComboBox}">
<Grid x:Name="templateRoot" SnapsToDevicePixels="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0"/>
</Grid.ColumnDefinitions>
<Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
<Themes:SystemDropShadowChrome x:Name="shadow" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=templateRoot}">
<Border x:Name="dropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
<ScrollViewer x:Name="DropDownScrollViewer">
<Grid x:Name="grid" RenderOptions.ClearTypeHint="Enabled">
<Canvas x:Name="canvas" HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="opaqueRect" Fill="{Binding Background, ElementName=dropDownBorder}" Height="{Binding ActualHeight, ElementName=dropDownBorder}" Width="{Binding ActualWidth, ElementName=dropDownBorder}"/>
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Grid>
</ScrollViewer>
</Border>
</Themes:SystemDropShadowChrome>
</Popup>
<ToggleButton x:Name="toggleButton" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ComboBoxToggleButton2}"/>
<Border x:Name="border" Background="{StaticResource TextBox.Static.Background}" Margin="{TemplateBinding BorderThickness}">
<TextBox x:Name="PART_EditableTextBox" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" IsReadOnly="{Binding IsReadOnly, RelativeSource={RelativeSource TemplatedParent}}" Margin="{TemplateBinding Padding}" Style="{StaticResource ComboBoxEditableTextBox}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Opacity" TargetName="border" Value="0.56"/>
</Trigger>
<Trigger Property="IsKeyboardFocusWithin" Value="true">
<Setter Property="Foreground" Value="Black"/>
</Trigger>
<Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
<Setter Property="Margin" TargetName="shadow" Value="0,0,5,5"/>
<Setter Property="Color" TargetName="shadow" Value="#71000000"/>
</Trigger>
<Trigger Property="HasItems" Value="false">
<Setter Property="Height" TargetName="dropDownBorder" Value="95"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsGrouping" Value="true"/>
<Condition Property="VirtualizingPanel.IsVirtualizingWhenGrouping" Value="false"/>
</MultiTrigger.Conditions>
<Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
</MultiTrigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="DropDownScrollViewer" Value="false">
<Setter Property="Canvas.Top" TargetName="opaqueRect" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="opaqueRect" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Style x:Key="ComboBoxStyleCustom" TargetType="{x:Type ComboBox}">
<Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisual}"/>
<Setter Property="Background" Value="{StaticResource ComboBox.Static.Background}"/>
<Setter Property="BorderBrush" Value="{StaticResource ComboBox.Static.Border}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="Padding" Value="6,3,5,3"/>
<Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
<Setter Property="ScrollViewer.PanningMode" Value="Both"/>
<Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
<Setter Property="Template" Value="{StaticResource ComboBoxTemplate}"/>
<Style.Triggers>
<Trigger Property="IsEditable" Value="true">
<Setter Property="IsTabStop" Value="false"/>
<Setter Property="Padding" Value="2"/>
<Setter Property="Template" Value="{StaticResource ComboBoxEditableTemplate}"/>
</Trigger>
</Style.Triggers>
</Style>
</Window.Resources>
<Grid>
<ComboBox HorizontalAlignment="Left" Margin="317.429,194.428,0,0" VerticalAlignment="Top" Width="120" Style="{DynamicResource ComboBoxStyleCustom}">
<ComboBox.Resources>
<!-- color of ComboBoxItem -->
<SolidColorBrush x:Key="{x:Static SystemColors.WindowBrushKey}" Color="#FF17FF00" />
</ComboBox.Resources>
<ComboBoxItem>One</ComboBoxItem>
<ComboBoxItem>Two</ComboBoxItem>
<ComboBoxItem>Three</ComboBoxItem>
</ComboBox>
</Grid>
</Window>

Just add the following to your ComboBox style :)
BasedOn="{StaticResource {x:Type ComboBox}}"
Example:
<Style TargetType="ComboBox" BasedOn="{StaticResource {x:Type ComboBox}}">
<Setter Property="Background" Value="Red" />
//Other setters, triggers, etc...
</Style>
Believe me, it's probably the shortest way to do it... I don't see why we have to completely re-define the ComboBox template.