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Opened ContextMenu -> RightClick triggers LeftClickCommand in ViewModel

I have a ContextMenu bound to an HierarchicalDataTemplate that displays a TreeView structure. The ContextMenu also is a HierarchicalDataTemplate that obtains its structure through an ObservableCollection of MenuItemViewModels.
My problem is, that when running the program a RightClick on an already opened ContextMenu unwantedly triggers a LeftClick on the MenuItem - why is that so? What am i missing? I am unable to find the error in code and would appreciate any help.
The XAML for the TreeView:
<HierarchicalDataTemplate x:Key="TreeHierarchicalDataTemplate" DataType="{x:Type model:TreeNodeBaseViewModel}" ItemsSource="{Binding Path=SubItems, UpdateSourceTrigger=PropertyChanged}" ItemContainerStyle="{StaticResource MenuItemTemplateItemContainerStyle}">
<!--...-->
</HierarchicalDataTemplate>
<Style x:Key="MenuItemTemplateItemContainerStyle" TargetType="{x:Type TreeViewItem}">
<Setter Property="ContextMenu" Value="{DynamicResource MenuItemContextMenu}"/>
<Setter Property="IsEnabled" Value="{Binding IsEnabled}"/>
<Setter Property="AllowDrop" Value="{Binding IsDropAllowed}"/>
<Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}"/>
<Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}"/>
<Setter Property="Tag" Value="{Binding}"/>
<Style.Triggers>
<DataTrigger Binding="{Binding Path=IsVisible}" Value="False">
<Setter Property="Visibility" Value="Collapsed"/>
</DataTrigger>
<Trigger Property="beh:TreeNodeMouseOver.IsMouseDirectlyOverItem" Value="True">
<Setter Property="Background" Value="AliceBlue" />
</Trigger>
</Style.Triggers>
</Style>
The XAML for the ContextMenu:
<ContextMenu x:Key="MenuItemContextMenu" ItemsSource="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}, Path=TreeViewModel.MenuItems, UpdateSourceTrigger=PropertyChanged}">
<ContextMenu.ItemTemplate>
<HierarchicalDataTemplate DataType="{x:Type model:MenuItemBaseViewModel}" ItemsSource="{Binding Path=SubItems, UpdateSourceTrigger=PropertyChanged}">
<TextBlock Text="{Binding Header}"/>
</HierarchicalDataTemplate>
</ContextMenu.ItemTemplate>
<ContextMenu.ItemContainerStyle>
<Style TargetType="MenuItem">
<Setter Property="ToolTip" Value="{Binding ToolTip}"/>
<Setter Property="IsHitTestVisible" Value="True"/>
<Setter Property="IsEnabled" Value="{Binding IsEnabled}"/>
<Setter Property="Command" Value="{Binding MouseLeftButtonDownCommand}"/> <!--LeftClick is bound to ViewModel here, but RightClick is not even defined anywhere in MenuItem or ViewModel-->
<Style.Triggers>
<DataTrigger Binding="{Binding VisibleSubItems}" Value="0">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
<Grid Margin="-1">
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="22" SharedSizeGroup="MenuItemIconColumnGroup" Width="Auto"/>
<ColumnDefinition Width="13"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="20"/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
<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"/>
<Border x:Name="GlyphPanel" BorderBrush="#FF26A0DA" BorderThickness="1" Background="#3D26A0DA" ClipToBounds="False" HorizontalAlignment="Center" Height="22" Margin="-1,0,0,0" Visibility="Hidden" VerticalAlignment="Center" Width="22">
<Path x:Name="Glyph" 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="#FF212121" FlowDirection="LeftToRight" Height="11" Width="10"/>
</Border>
<ContentPresenter x:Name="menuHeaderContainer" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.Column="2" ContentStringFormat="{TemplateBinding HeaderStringFormat}" 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"/>
<Button Grid.Column="5" Margin="2" Command="{Binding EditCommand}" Visibility="{Binding IsEditVisible, Converter={StaticResource BooleanToVisibilityConverter}}" Background="Transparent" BorderThickness="0">
<iconPacks:PackIconMaterialLight Width="14" Height="14" Foreground="Black" HorizontalAlignment="Center" VerticalAlignment="Center" Kind="Cog" />
</Button>
<Button Grid.Column="6" Margin="2" Command="{Binding HelpCommand}" Visibility="{Binding IsInfoVisible, Converter={StaticResource BooleanToVisibilityConverter}}" Background="Transparent" BorderThickness="0">
<iconPacks:PackIconMaterial Width="14" Height="14" Foreground="Blue" HorizontalAlignment="Center" VerticalAlignment="Center" Kind="InformationOutline" />
</Button>
</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="#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="Glyph" Value="#FF707070"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsHighlighted" Value="True"/>
<Condition Property="IsEnabled" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="#0A000000"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#21000000"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</DataTrigger>
<DataTrigger Binding="{Binding IsVisible}" Value="false">
<Setter Property="MenuItem.Visibility" Value="Collapsed"/>
</DataTrigger>
<DataTrigger Binding="{Binding PackIcon.IconType}" Value="{x:Type iconPacks:PackIconSimpleIconsKind}">
<Setter Property="MenuItem.Icon" Value="{StaticResource Simple}"/>
</DataTrigger>
<DataTrigger Binding="{Binding PackIcon.IconType}" Value="{x:Type iconPacks:PackIconEntypoKind}">
<Setter Property="MenuItem.Icon" Value="{StaticResource Entypo}"/>
</DataTrigger>
<DataTrigger Binding="{Binding PackIcon.IconType}" Value="{x:Type iconPacks:PackIconFontAwesomeKind}">
<Setter Property="MenuItem.Icon" Value="{StaticResource FontAwesome}"/>
</DataTrigger>
<DataTrigger Binding="{Binding PackIcon.IconType}" Value="{x:Type iconPacks:PackIconMaterialKind}">
<Setter Property="MenuItem.Icon" Value="{StaticResource Material}"/>
</DataTrigger>
<DataTrigger Binding="{Binding PackIcon.IconType}" Value="{x:Type iconPacks:PackIconMaterialLightKind}">
<Setter Property="MenuItem.Icon" Value="{StaticResource MaterialLight}"/>
</DataTrigger>
<DataTrigger Binding="{Binding PackIcon.IconType}" Value="{x:Type iconPacks:PackIconModernKind}">
<Setter Property="MenuItem.Icon" Value="{StaticResource Modern}"/>
</DataTrigger>
<DataTrigger Binding="{Binding PackIcon.IconType}" Value="{x:Type iconPacks:PackIconOcticonsKind}">
<Setter Property="MenuItem.Icon" Value="{StaticResource Octicons}"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ContextMenu.ItemContainerStyle>
<i:Interaction.Behaviors>
<beh:EventToCommandBehavior Command="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}, Path=TreeViewModel.ContextMenuOpenedCommand}" Event="Opened" PassArguments="True" />
</i:Interaction.Behaviors>
</ContextMenu>

That behavior is by design. If you want seperate left/right click handlers for your menu items then you need to add the corresponding MouseBindings to your MenuItem InputBindings collection. If you need to do that in a style then you can use the Attach behavior in this question.
<ContextMenu.ItemContainerStyle>
<Style TargetType="MenuItem">
<Setter Property="behaviors:Attach.InputBindings">
<Setter.Value>
<InputBindingCollection>
<MouseBinding Gesture="LeftClick" Command="{Binding LeftClickCommand}" />
<MouseBinding Gesture="RightClick" Command="{Binding RightClickCommand}" />
</InputBindingCollection>
</Setter.Value>
</Setter>
</Style>
</ContextMenu.ItemContainerStyle>

WPF combobox prevent selection update

I want to be able to select a value in a ComboBox but avoid the selected value updating.
In other words, for the picture above, I want the empty selection to remain empty when one of the items is selected.
I've tried a ton of different solutions - various bindings on SelectedIndex, SelectedValue, SelectionChanged, playing with IsEditable, IsReadonly, IsHitTestVisible, Text, etc, etc, etc.
How can I prevent the combobox from automatically updating the text on selection, but still have it able to select a choice?
Sample code:
<ComboBox
IsReadOnly="True"
IsEditable="False"
Text="..."
>
<ComboBoxItem>
<TextBlock>Close</TextBlock>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock>Delete</TextBlock>
</ComboBoxItem>
</ComboBox>

As an experienced WPF developer, I'll put some light on this problem.
Basically, you have tried a ton of different solutions using the wrong instruments.
In WPF each control has a default style, which fully describes what you'll see on the screen (nothing hidden here). What is not that obvious, however, is how to take a look at the default style of a control.
(I work with VS 2013, but the steps described below should work in later versions as well)
Create a new project in 'Blend for Visual Studio 2013"
In the "Assets" tab search box type "ComboBox" and double click on it
Right click on the ComboBox you've just added in the "Objects and timeline" panel and select "Edit template -> Edit a copy..."
Switch to code view (the button is on the top right side)
Yeah, the default styles are quite lengthy!
Now, what we are interested in is the ControlTemplate. There must be some control in this tempplate responsible for displaying the selected item. If you are in a hurry, just find it and delete the appropriate binding.
Below is an example solution, which I would call "Couple hours before deadline solution". Read the comments in the XAML for the proper approach.
<Window x:Class="ComboBoxAdventures.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero2"
Title="MainWindow" Height="350" Width="525">
<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>
<LinearGradientBrush x:Key="ComboBox.Static.Background" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFF0F0F0" Offset="0.0"/>
<GradientStop Color="#FFE5E5E5" Offset="1.0"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="ComboBox.Static.Border" Color="#FFACACAC"/>
<SolidColorBrush x:Key="ComboBox.Static.Editable.Background" Color="#FFFFFFFF"/>
<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"/>
<LinearGradientBrush x:Key="ComboBox.MouseOver.Background" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFECF4FC" Offset="0.0"/>
<GradientStop Color="#FFDCECFC" Offset="1.0"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="ComboBox.MouseOver.Border" Color="#FF7EB4EA"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Editable.Background" Color="#FFFFFFFF"/>
<SolidColorBrush x:Key="ComboBox.MouseOver.Editable.Border" Color="#FF7EB4EA"/>
<LinearGradientBrush x:Key="ComboBox.MouseOver.Editable.Button.Background" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFEBF4FC" Offset="0.0"/>
<GradientStop Color="#FFDCECFC" Offset="1.0"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="ComboBox.MouseOver.Editable.Button.Border" Color="#FF7EB4EA"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Glyph" Color="#FF000000"/>
<LinearGradientBrush x:Key="ComboBox.Pressed.Background" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFDAECFC" Offset="0.0"/>
<GradientStop Color="#FFC4E0FC" Offset="1.0"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="ComboBox.Pressed.Border" Color="#FF569DE5"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Editable.Background" Color="#FFFFFFFF"/>
<SolidColorBrush x:Key="ComboBox.Pressed.Editable.Border" Color="#FF569DE5"/>
<LinearGradientBrush x:Key="ComboBox.Pressed.Editable.Button.Background" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFDAEBFC" Offset="0.0"/>
<GradientStop Color="#FFC4E0FC" Offset="1.0"/>
</LinearGradientBrush>
<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="#FFF0F0F0"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Border" Color="#FFD9D9D9"/>
<SolidColorBrush x:Key="ComboBox.Disabled.Editable.Background" Color="#FFFFFFFF"/>
<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="ComboBoxToggleButton" 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 ComboBoxToggleButton}"/>
<!-- This content presenter displays the selected item.
The easiest way is just to remove the Content binding as shown below.
The proper way to do those kind of things, however, is to inherit from ComboBox,
define a dependency property ShowSelectedItem and then add a trigger for the ContentPreseneter below,
which changes the Content based on the value of the ShowSelectedItem.
You'll also need a custom IValueConverter for mapping bool to Hidden/Visible and some
googling about Generic.xaml and how to overwrite the DefaultStyle dependency property in the static constructor.
You may also wonder why I didn't just comment out the whole ContentPresenter.
Well, lets say it just keeps the 'backbone' of the control :)
-->
<ContentPresenter x:Name="contentPresenter" Tag="I have removed only this part: Content='{TemplateBinding SelectionBoxItem}'"
ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
Content="" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
IsHitTestVisible="false" Margin="{TemplateBinding Padding}"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
</ContentPresenter>
</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 ComboBoxToggleButton}"/>
<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="Style.ComboBox.Ghost" 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>
<StackPanel>
<ComboBox Style="{StaticResource Style.ComboBox.Ghost}" ItemsSource="{Binding SelectedDummyModel}" SelectedItem="{Binding DummyModels}"/>
</StackPanel>
</Window>

Here's something I came up with based on adding buttons to the combobox.
This is just two buttons added to the combobox.
<ComboBox x:Name="cboBox" HorizontalAlignment="Left" VerticalAlignment="Top" Width="151" Margin="182,132,0,0" SelectionChanged="cboBox_SelectionChanged" Grid.Column="1" Height="22">
<Button Content="Close" Width="141" Click="Button_Close" Height="51"/>
<Button Content="Delete" Width="141" Click="Button_Delete" Height="51"/>
</ComboBox>
I then went into the code behind and added code to handle the click event for each button.
As you will see we aren't even dealing with the combobox other than using it to show the buttons and then closing the dropdown upon clicking one of the buttons.
private void Button_Close(object sender, RoutedEventArgs e)
{
lbl1.Content = "Close Clicked";
cboBox.IsDropDownOpen = false;
}
private void Button_Delete(object sender, RoutedEventArgs e)
{
lbl1.Content = "Delete Clicked";
cboBox.IsDropDownOpen = false;
}
I really need to get better versed with WPF. I actually enjoyed messing with this.

set combo box selected item value to null through data trigger

I am trying to set Combobox Selected Item property to null through Control Template I've added Checkbox so I can use datatrigger and set SelectedItem to null when Checkbox isChecked is true. Basically Im trying to add clear filter checkbox that will show up when SelectedItem has some value, and upon clicking this checkbox selected item will null itself
PS.Initially I was going to use button instead of checkbox but Im not sure how to wire it up all inside Dictionary
<CheckBox Panel.ZIndex="2" Width="20" HorizontalAlignment="Left" Foreground="Red" FontWeight="Bold" Background="Red" Content="X" x:Name="chkBOX">
<CheckBox.Style>
<Style TargetType="CheckBox">
<Style.Setters>
<Setter Property="Visibility" Value="Visible"/>
<Setter Property="IsEnabled" Value="True"/>
<Setter Property="VerticalAlignment" Value="Center"/>
</Style.Setters>
<Style.Triggers>
<!-- If selected item is null hide toggle button -->
<DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}, Path=SelectedItem}" Value="{x:Null}">
<Setter Property="Visibility" Value="Hidden"/>
</DataTrigger>
</Style.Triggers>
</Style>
</CheckBox.Style>
</CheckBox>
And now Im looking to interact with it so selected item goes null when checkbox is checked
Hope thats ok :)
EDIT:
I've made SOME progress
I can null selected item with below
however this only works if Selected item is not binded on my window form IE
this works
<ComboBox Grid.Row="1" Margin="10,0,0,0" ItemsSource="{Binding Users}" Tag="Select Full Name"
DisplayMemberPath="FullName" Template="{DynamicResource ComboBoxWithPlaceholder}" />
this doesnt work
<ComboBox Grid.Row="1" Margin="10,0,0,0" ItemsSource="{Binding Users}" Tag="Select Full Name"
DisplayMemberPath="FullName" SelectedItem="{Binding SelectedUserFullName}" Template="{DynamicResource ComboBoxWithPlaceholder}" />

Nullable combobox with placeholeder
Link to gif
<ControlTemplate x:Key="ComboBoxWithPlaceholder" x:Name="cb" TargetType="{x:Type ComboBox}">
<Grid x:Name="templateRoot" SnapsToDevicePixels="True">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="16"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0"/>
</Grid.ColumnDefinitions>
<ToggleButton Panel.ZIndex="2" Width="16" HorizontalAlignment="Left" Foreground="Red" ToolTip="Clear Selection" Opacity="0.5"
FontWeight="Bold" Background="Transparent" BorderThickness="0" Content="X" x:Name="toggleB">
<ToggleButton.Style>
<Style TargetType="ToggleButton">
<Style.Setters>
<Setter Property="VerticalAlignment" Value="Center"/>
</Style.Setters>
<Style.Triggers>
<!-- If selected item is null hide toggle button -->
<DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}, Path=SelectedItem}" Value="{x:Null}">
<Setter Property="Visibility" Value="Collapsed"/>
<!-- When hidden ensure its not checked -->
<Setter Property="IsChecked" Value="False"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ToggleButton.Style>
</ToggleButton>
<!-- Placeholder textblock text is bound to tag property of combobox -->
<TextBlock Grid.ColumnSpan="2" Grid.Column="0" x:Name="tb" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Foreground="DimGray"
IsHitTestVisible="False" Panel.ZIndex="1" Text="{Binding Tag,RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}}">
<TextBlock.Style>
<Style TargetType="TextBlock">
<Style.Setters>
<Setter Property="Visibility" Value="Hidden"/>
<Setter Property="Margin" Value="5,0"/>
<Setter Property="VerticalAlignment" Value="{Binding VerticalContentAlignment,RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}}"/>
</Style.Setters>
<Style.Triggers>
<DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}, Path=SelectedItem}" Value="{x:Null}">
<Setter Property="Visibility" Value="Visible"/>
</DataTrigger>
</Style.Triggers>
</Style>
</TextBlock.Style>
</TextBlock>
<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}" VerticalContentAlignment="Center"
Background="{TemplateBinding Background}" Grid.Column="0" Grid.ColumnSpan="3" 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="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border x:Name="templateRoot" BorderBrush="#FFACACAC" BorderThickness="{TemplateBinding BorderThickness}" SnapsToDevicePixels="True">
<Border.Background>
<LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFF0F0F0" Offset="0"/>
<GradientStop Color="#FFE5E5E5" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<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="F1M0,0L2.667,2.66665 5.3334,0 5.3334,-1.78168 2.6667,0.88501 0,-1.78168 0,0z" Fill="#FF606060" HorizontalAlignment="Center" Margin="0" VerticalAlignment="Center"/>
</Border>
</Border>
<ControlTemplate.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, 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="White"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#FFABADB3"/>
<Setter Property="Background" TargetName="splitBorder" Value="Transparent"/>
<Setter Property="BorderBrush" TargetName="splitBorder" Value="Transparent"/>
</MultiDataTrigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Fill" TargetName="Arrow" Value="Black"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type ComboBox}}}" Value="false"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot">
<Setter.Value>
<LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFECF4FC" Offset="0"/>
<GradientStop Color="#FFDCECFC" Offset="1"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#FF7EB4EA"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type ComboBox}}}" Value="true"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="White"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#FF7EB4EA"/>
<Setter Property="Background" TargetName="splitBorder">
<Setter.Value>
<LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFEBF4FC" Offset="0"/>
<GradientStop Color="#FFDCECFC" Offset="1"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="BorderBrush" TargetName="splitBorder" Value="#FF7EB4EA"/>
</MultiDataTrigger>
<Trigger Property="IsPressed" Value="True">
<Setter Property="Fill" TargetName="Arrow" Value="Black"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type ComboBox}}}" Value="false"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot">
<Setter.Value>
<LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFDAECFC" Offset="0"/>
<GradientStop Color="#FFC4E0FC" Offset="1"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#FF569DE5"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type ComboBox}}}" Value="true"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="White"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#FF569DE5"/>
<Setter Property="Background" TargetName="splitBorder">
<Setter.Value>
<LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFDAEBFC" Offset="0"/>
<GradientStop Color="#FFC4E0FC" Offset="1"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="BorderBrush" TargetName="splitBorder" Value="#FF569DE5"/>
</MultiDataTrigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Fill" TargetName="Arrow" Value="#FFBFBFBF"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type ComboBox}}}" Value="false"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="#FFF0F0F0"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#FFD9D9D9"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding IsEditable, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type ComboBox}}}" Value="true"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="White"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#FFBFBFBF"/>
<Setter Property="Background" TargetName="splitBorder" Value="Transparent"/>
<Setter Property="BorderBrush" TargetName="splitBorder" Value="Transparent"/>
</MultiDataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ToggleButton.Style>
</ToggleButton>
<ContentPresenter Grid.Column="1" 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}">
</ContentPresenter>
</Grid>
<ControlTemplate.Triggers>
<!-- If selected set selected item to null-->
<DataTrigger Binding="{Binding IsPressed, ElementName=toggleB}" Value="True">
<Setter Property="SelectedValue" Value="{x:Null}"/>
<Setter Property="SelectedValuePath" Value="{x:Null}"/>
<Setter Property="SelectedItem" Value="{x:Null}"/>
</DataTrigger>
<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="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>
bit I was missing is to set SelectedValuePath and SelectedValue to null as well as Selected item
<DataTrigger Binding="{Binding IsPressed, ElementName=toggleB}" Value="True">
<Setter Property="SelectedValue" Value="{x:Null}"/>
<Setter Property="SelectedValuePath" Value="{x:Null}"/>
<Setter Property="SelectedItem" Value="{x:Null}"/>
</DataTrigger>

How to change Datagrid row background when clicking on togglebutton inside rowheader?

I am overriding DataGrid row header style, and I’m been hindered with the following two issues:
1.The DataGrid row header contains an Image and a ToggleButton. When the ToggleButton is ‘Checked’ I want to change the background color of the whole row, however I only managed to change the background of the row header and couldn’t figure out a way to fire a trigger on the row level.
2.When a row is selected the row and the header background is set to different background, the other way around does not work. What I need to do is to change the background of the row when the row header is selected
Here is a the style code of the DataGridRowHeader
<Style x:Key="{x:Type DataGridRowHeader}" TargetType="{x:Type DataGridRowHeader}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type DataGridRowHeader}">
<Grid>
<Border Name="RowHeaderBorder"
BorderThickness="0,0,3,0"
Margin="0,0,0,0"
BorderBrush="{StaticResource DataGridRowBorder}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="50"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Image Grid.Column="0" Source="{Binding ImageSource}" RenderOptions.BitmapScalingMode="HighQuality" Stretch="None" VerticalAlignment="Center" HorizontalAlignment="Center"/>
<ToggleButton x:Name="tglButton" Visibility="{Binding ActiveCall}" Grid.Column="1" Content="Button" Focusable="True" BorderThickness="1" Width="80" Height="33" VerticalAlignment="Top" >
</ToggleButton>
</Grid>
</Border>
</Grid>
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding ElementName=tglButton, Path=IsChecked}" Value="true">
<Setter Property="Background" TargetName="RowHeaderBorder" Value ="Green"/>
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
And here is the style of the DataGridRow:
<Style x:Key="DataGridRowStyle" TargetType="{x:Type DataGridRow}">
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="BorderBrush" Value="{StaticResource DataGridRowBorder}"/>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Background" Value="{StaticResource DataGridRowHoveredBackground}" />
<Setter Property="FontWeight" Value="Bold"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Effect">
<Setter.Value>
<DropShadowEffect ShadowDepth="0" BlurRadius="20"/>
</Setter.Value>
</Setter>
</Trigger>
<Trigger Property="IsSelected" Value="true">
<Setter Property="Foreground" Value="Black"/>
<Setter Property="Background" Value="{StaticResource RowBackgroundSelectedBrush2}" />
<Setter Property="FontWeight" Value="Bold"/>
<Setter Property="Effect">
<Setter.Value>
<DropShadowEffect ShadowDepth="0" BlurRadius="20"/>
</Setter.Value>
</Setter>
</Trigger>
<DataTrigger Binding="{Binding ElementName=DataGridRowHeader.tglButton, Path=IsChecked}" Value="true">
<Setter Property="Background" Value ="Green"/>
</DataTrigger>
</Style.Triggers>
</Style>

This is because tglButton is present in the row header, which is a child of the DataGridRow. A trigger defined in child row header is trying to update a property of the parent (DataGridRow) and is not able to find it.
One solution for this is to define the template for DataGridRowHeader inside the template for DataGridRow and supply a name for it, which can be used in the trigger. A very crude example:
<Style
TargetType="{x:Type DataGridRow}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type DataGridRow}">
<Border x:Name="DGR_Border"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Background="{TemplateBinding Background}"
SnapsToDevicePixels="True"
CornerRadius="8,8,8,8">
<SelectiveScrollingGrid>
<SelectiveScrollingGrid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
</SelectiveScrollingGrid.ColumnDefinitions>
<SelectiveScrollingGrid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
</SelectiveScrollingGrid.RowDefinitions>
<DataGridCellsPresenter Grid.Column="1"
ItemsPanel="{TemplateBinding ItemsPanel}"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
<DataGridDetailsPresenter Grid.Column="1"
Grid.Row="1"
SelectiveScrollingGrid.SelectiveScrollingOrientation="{Binding AreRowDetailsFrozen,
ConverterParameter={x:Static SelectiveScrollingOrientation.Vertical},
Converter={x:Static DataGrid.RowDetailsScrollingConverter},
RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"
Visibility="{TemplateBinding DetailsVisibility}" />
<DataGridRowHeader Name="RHeader" Grid.RowSpan="2"
SelectiveScrollingGrid.SelectiveScrollingOrientation="Vertical"
Visibility="{Binding HeadersVisibility,
ConverterParameter={x:Static DataGridHeadersVisibility.Row},
Converter={x:Static DataGrid.HeadersVisibilityConverter},
RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="50" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<ToggleButton x:Name="tglButton"
Grid.Column="1"
Content="Button"
Focusable="True"
BorderThickness="1"
Width="80"
Height="33"
VerticalAlignment="Top">
</ToggleButton>
</Grid>
</DataGridRowHeader>
</SelectiveScrollingGrid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSelected"
Value="True">
<Setter TargetName="DGR_Border"
Property="Background"
Value="Gray" />
</Trigger>
<DataTrigger Binding="{Binding ElementName=tglButton, Path=IsChecked}"
Value="true">
<Setter Property="Background"
TargetName="RHeader"
Value="Green" />
<Setter Property="Background"
Value="Green" />
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="DataContext"
Value="{Binding RelativeSource={RelativeSource Self}}" />
<Setter Property="BorderThickness"
Value="1" />
<Setter Property="BorderBrush"
Value="Aqua" />
<Style.Triggers>
<Trigger Property="IsSelected"
Value="true">
<Setter Property="Foreground"
Value="Black" />
<Setter Property="Background"
Value="Pink" />
<Setter Property="FontWeight"
Value="Bold" />
<Setter Property="Effect">
<Setter.Value>
<DropShadowEffect ShadowDepth="0"
BlurRadius="20" />
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>

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.