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Combobox Foreground when expanded in UWP

I have the following code in which I am setting foreground for combobox item in template.
<ComboBox Height="100" ItemsSource="{Binding Data}" BorderBrush="Black" BorderThickness="0.5" >
<ComboBox.ItemTemplate>
<DataTemplate >
<TextBlock Text="{Binding}" Foreground="Black" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
I just need to change the foreground color when the combobox is expanded (i.e when it has more items). Please refer the screenshot.
At this time I need the foreground to be white color. Please suggest!

Try setting the foreground of ComboBox to white.And the item template foreground also. Should work!

To change background of open combobox you need to edit it's template. Bellow I've changed it to red for example. (Check last "Popup" section of the template)
<Style x:Key="ComboBoxStyle1" TargetType="ComboBox">
<Setter Property="Padding" Value="12,5,0,7"/>
<Setter Property="MinWidth" Value="{ThemeResource ComboBoxThemeMinWidth}"/>
<Setter Property="Foreground" Value="{ThemeResource SystemControlForegroundBaseHighBrush}"/>
<Setter Property="Background" Value="{ThemeResource SystemControlBackgroundAltMediumLowBrush}"/>
<Setter Property="BorderBrush" Value="{ThemeResource SystemControlForegroundBaseMediumLowBrush}"/>
<Setter Property="BorderThickness" Value="{ThemeResource ComboBoxBorderThemeThickness}"/>
<Setter Property="TabNavigation" Value="Once"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Disabled"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.HorizontalScrollMode" Value="Disabled"/>
<Setter Property="ScrollViewer.VerticalScrollMode" Value="Auto"/>
<Setter Property="ScrollViewer.IsVerticalRailEnabled" Value="True"/>
<Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False"/>
<Setter Property="ScrollViewer.BringIntoViewOnFocusChange" Value="True"/>
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
<Setter Property="HorizontalAlignment" Value="Left"/>
<Setter Property="VerticalAlignment" Value="Top"/>
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}"/>
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<CarouselPanel/>
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="32"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlPageBackgroundAltMediumBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightBaseMediumBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundListMediumBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightBaseMediumLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="PlaceholderTextBlock">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="DropDownGlyph">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="HighlightBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightTransparentBrush}"/>
</ObjectAnimationUsingKeyFrames>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="HighlightBackground"/>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="PlaceholderTextBlock">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="DropDownGlyph">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseMediumHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="FocusedPressed">
<Storyboard>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="HighlightBackground"/>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="PlaceholderTextBlock">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="DropDownGlyph">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseMediumHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Unfocused"/>
<VisualState x:Name="PointerFocused"/>
<VisualState x:Name="FocusedDropDown">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetProperty="Visibility" Storyboard.TargetName="PopupBorder">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="DropDownStates">
<VisualState x:Name="Opened">
<Storyboard>
<SplitOpenThemeAnimation ClosedTargetName="ContentPresenter" OffsetFromCenter="{Binding TemplateSettings.DropDownOffset, RelativeSource={RelativeSource Mode=TemplatedParent}}" OpenedTargetName="PopupBorder" OpenedLength="{Binding TemplateSettings.DropDownOpenedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Closed">
<Storyboard>
<SplitCloseThemeAnimation ClosedTargetName="ContentPresenter" OffsetFromCenter="{Binding TemplateSettings.DropDownOffset, RelativeSource={RelativeSource Mode=TemplatedParent}}" OpenedTargetName="PopupBorder" OpenedLength="{Binding TemplateSettings.DropDownOpenedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<ContentPresenter x:Name="HeaderContentPresenter" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" FontWeight="{ThemeResource ComboBoxHeaderThemeFontWeight}" FlowDirection="{TemplateBinding FlowDirection}" Margin="{ThemeResource ComboBoxHeaderThemeMargin}" Visibility="Collapsed" x:DeferLoadStrategy="Lazy"/>
<Border x:Name="Background" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" Grid.Row="1"/>
<Border x:Name="HighlightBackground" BorderBrush="{ThemeResource SystemControlHighlightBaseMediumLowBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{ThemeResource SystemControlHighlightListAccentLowBrush}" Grid.ColumnSpan="2" Opacity="0" Grid.Row="1"/>
<ContentPresenter x:Name="ContentPresenter" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" Grid.Row="1" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
<TextBlock x:Name="PlaceholderTextBlock" Foreground="{ThemeResource SystemControlPageTextBaseHighBrush}" FontWeight="{ThemeResource ComboBoxPlaceholderTextThemeFontWeight}" Text="{TemplateBinding PlaceholderText}"/>
</ContentPresenter>
<FontIcon x:Name="DropDownGlyph" AutomationProperties.AccessibilityView="Raw" Grid.Column="1" Foreground="{ThemeResource SystemControlForegroundBaseMediumHighBrush}" FontSize="12" FontFamily="{ThemeResource SymbolThemeFontFamily}" Glyph="" HorizontalAlignment="Right" IsHitTestVisible="False" Margin="0,10,10,10" Grid.Row="1" VerticalAlignment="Center"/>
<Popup x:Name="Popup">
<Border x:Name="PopupBorder" BorderBrush="{ThemeResource SystemControlForegroundBaseMediumHighBrush}" BorderThickness="{ThemeResource ComboBoxDropdownBorderThickness}" Background="Red" HorizontalAlignment="Stretch">
<ScrollViewer x:Name="ScrollViewer" AutomationProperties.AccessibilityView="Raw" BringIntoViewOnFocusChange="{TemplateBinding ScrollViewer.BringIntoViewOnFocusChange}" Foreground="{ThemeResource SystemControlForegroundBaseHighBrush}" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" MinWidth="{ThemeResource ComboBoxPopupThemeMinWidth}" VerticalSnapPointsType="OptionalSingle" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" VerticalSnapPointsAlignment="Near" ZoomMode="Disabled">
<ItemsPresenter/>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Center placeholder text in TextBox

I have multiple TextBoxes in my XAML-Page.
<TextBox Name="TxtCompanyId" PlaceholderText="FirmaId" Height="40" Margin="5" TextAlignment="Center" />
The TextAlignment="Center" property only centers the text of the TextBox and not the placeholdertext/hint text. Is there a way to center both?

You can surely modify the defalut style and set PlaceholderTextContentPresenter's horizontal alignment to Center:
<ContentControl x:Name="PlaceholderTextContentPresenter" Grid.ColumnSpan="2" Content="{TemplateBinding PlaceholderText}" Foreground="{ThemeResource SystemControlPageTextBaseMediumBrush}" IsHitTestVisible="False" IsTabStop="False" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" HorizontalAlignment="Center"/>
The complete modified style:
<Page.Resources>
<Style x:Key="MyTextBoxStyl" TargetType="TextBox">
<Setter Property="MinWidth" Value="{ThemeResource TextControlThemeMinWidth}"/>
<Setter Property="MinHeight" Value="{ThemeResource TextControlThemeMinHeight}"/>
<Setter Property="Foreground" Value="{ThemeResource SystemControlForegroundBaseHighBrush}"/>
<Setter Property="Background" Value="{ThemeResource SystemControlBackgroundAltHighBrush}"/>
<Setter Property="BorderBrush" Value="{ThemeResource SystemControlForegroundChromeDisabledLowBrush}"/>
<Setter Property="SelectionHighlightColor" Value="{ThemeResource SystemControlHighlightAccentBrush}"/>
<Setter Property="BorderThickness" Value="{ThemeResource TextControlBorderThemeThickness}"/>
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Hidden"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Hidden"/>
<Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False"/>
<Setter Property="Padding" Value="{ThemeResource TextControlThemePadding}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Grid>
<Grid.Resources>
<Style x:Name="DeleteButtonStyle" TargetType="Button">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid x:Name="ButtonLayoutGrid" BorderBrush="{ThemeResource TextBoxButtonBorderThemeBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{ThemeResource TextBoxButtonBackgroundThemeBrush}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="GlyphElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="ButtonLayoutGrid">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="GlyphElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltChromeWhiteBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" To="0" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="ButtonLayoutGrid"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<TextBlock x:Name="GlyphElement" AutomationProperties.AccessibilityView="Raw" Foreground="{ThemeResource SystemControlForegroundChromeBlackMediumBrush}" FontStyle="Normal" FontSize="12" FontFamily="{ThemeResource SymbolThemeFontFamily}" HorizontalAlignment="Center" Text="" VerticalAlignment="Center"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="HeaderContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledTransparentBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledChromeDisabledLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="PlaceholderTextContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledChromeDisabledLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightChromeAltLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundHoverOpacity}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="PlaceholderTextContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlPageTextChromeBlackMediumLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundChromeWhiteBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundFocusedOpacity}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlForegroundChromeBlackHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="RequestedTheme" Storyboard.TargetName="ContentElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="Light"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="ButtonStates">
<VisualState x:Name="ButtonVisible">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="DeleteButton">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="ButtonCollapsed"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="BackgroundElement" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" Margin="{TemplateBinding BorderThickness}" Opacity="{ThemeResource TextControlBackgroundRestOpacity}" Grid.Row="1" Grid.RowSpan="1"/>
<Border x:Name="BorderElement" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Grid.ColumnSpan="2" Grid.Row="1" Grid.RowSpan="1"/>
<ContentPresenter x:Name="HeaderContentPresenter" Grid.ColumnSpan="2" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Foreground="{ThemeResource SystemControlForegroundBaseHighBrush}" FontWeight="Normal" Margin="0,0,0,8" Grid.Row="0" Visibility="Collapsed" x:DeferLoadStrategy="Lazy"/>
<ScrollViewer x:Name="ContentElement" AutomationProperties.AccessibilityView="Raw" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" IsTabStop="False" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" ZoomMode="Disabled"/>
<!--Here below we modify the horizontal alignment-->
<ContentControl x:Name="PlaceholderTextContentPresenter" Grid.ColumnSpan="2" Content="{TemplateBinding PlaceholderText}" Foreground="{ThemeResource SystemControlPageTextBaseMediumBrush}" IsHitTestVisible="False" IsTabStop="False" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" HorizontalAlignment="Center"/>
<Button x:Name="DeleteButton" BorderThickness="{TemplateBinding BorderThickness}" Grid.Column="1" FontSize="{TemplateBinding FontSize}" IsTabStop="False" Margin="{ThemeResource HelperButtonThemePadding}" MinWidth="34" Grid.Row="1" Style="{StaticResource DeleteButtonStyle}" Visibility="Collapsed" VerticalAlignment="Stretch"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Page.Resources>
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<TextBox Name="TxtCompanyId" PlaceholderText="FirmaId" Height="40" Margin="5" TextAlignment="Center" Style="{StaticResource MyTextBoxStyl}"/>
</Grid>

How to apply global transform rules in app.xaml in windows phone 8

I want to apply this transform rule to all checkboxs:
<CheckBox.RenderTransform>
<ScaleTransform ScaleX="0.7" ScaleY="0.7" />
</CheckBox.RenderTransform>
Is there some way that I can put it into app.xaml so that I don't need to write this rule to every checkbox?

Sure, all you have to do is override default checkbox style. Add this code to your App.xaml file in <Application.Resources> section:
<!--Application Resources-->
<Application.Resources>
<Style x:Key="PhoneButtonBase" TargetType="ButtonBase">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderBrush" Value="{StaticResource PhoneForegroundBrush}"/>
<Setter Property="Foreground" Value="{StaticResource PhoneForegroundBrush}"/>
<Setter Property="BorderThickness" Value="{StaticResource PhoneBorderThickness}"/>
<Setter Property="FontFamily" Value="{StaticResource PhoneFontFamilySemiBold}"/>
<Setter Property="FontSize" Value="{StaticResource PhoneFontSizeMediumLarge}"/>
<Setter Property="Padding" Value="10,3,10,5"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ButtonBase">
<Grid Background="Transparent">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver"/>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentContainer">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneBackgroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneForegroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneForegroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentContainer">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="Transparent"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="ButtonBackground" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="0" Margin="{StaticResource PhoneTouchTargetOverhang}">
<ContentControl x:Name="ContentContainer" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Padding="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="PhoneRadioButtonCheckBoxBase" BasedOn="{StaticResource PhoneButtonBase}" TargetType="ToggleButton">
<Setter Property="Background" Value="{StaticResource PhoneRadioCheckBoxBrush}"/>
<Setter Property="BorderBrush" Value="{StaticResource PhoneRadioCheckBoxBrush}"/>
<Setter Property="FontSize" Value="{StaticResource PhoneFontSizeMedium}"/>
<Setter Property="FontFamily" Value="{StaticResource PhoneFontFamilyNormal}"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Padding" Value="0"/>
</Style>
<Style BasedOn="{StaticResource PhoneRadioButtonCheckBoxBase}" TargetType="CheckBox">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="CheckBox">
<Grid Background="Transparent">
<Grid.RenderTransform>
<ScaleTransform ScaleX="0.7" ScaleY="0.7" />
</Grid.RenderTransform>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver"/>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="CheckBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxPressedBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="CheckBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxPressedBorderBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="CheckMark">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxCheckBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="IndeterminateMark">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxCheckBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="CheckBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="CheckBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="CheckMark">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxCheckDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="IndeterminateMark">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxCheckDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentContainer">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="CheckStates">
<VisualState x:Name="Checked">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="CheckMark">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Unchecked"/>
<VisualState x:Name="Indeterminate">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="IndeterminateMark">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid Margin="{StaticResource PhoneTouchTargetLargeOverhang}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="32"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Border x:Name="CheckBackground" BorderBrush="{TemplateBinding Background}" BorderThickness="{StaticResource PhoneBorderThickness}" Background="{TemplateBinding Background}" HorizontalAlignment="Left" Height="32" IsHitTestVisible="False" VerticalAlignment="Center" Width="32"/>
<Rectangle x:Name="IndeterminateMark" Fill="{StaticResource PhoneRadioCheckBoxCheckBrush}" HorizontalAlignment="Center" Height="16" IsHitTestVisible="False" Grid.Row="0" Visibility="Collapsed" VerticalAlignment="Center" Width="16"/>
<Path x:Name="CheckMark" Data="M0,119 L31,92 L119,185 L267,0 L300,24 L122,250 z" Fill="{StaticResource PhoneRadioCheckBoxCheckBrush}" HorizontalAlignment="Center" Height="18" IsHitTestVisible="False" Stretch="Fill" StrokeThickness="2" StrokeLineJoin="Round" Visibility="Collapsed" VerticalAlignment="Center" Width="24"/>
<ContentControl x:Name="ContentContainer" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Grid.Column="1" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="12,0,0,0" Padding="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Grid>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Application.Resources>
I have added scale transformation in checkbox template:
<Grid.RenderTransform>
<ScaleTransform ScaleX="0.7" ScaleY="0.7" />
</Grid.RenderTransform>
I know, it's a lot of XAML code. But you can easily change any aspect of the checkbox control here.

C# XAML deactivate highlighting on tap

I want to deactivate the blue highlight on Windows Phone 8.1 that I get whenever I click a Button or ListBox Item. Like everything that is transparent is blue.
The Button is only displayed as a clickable image:
<Button Content="" BorderBrush="Transparent" Foreground="Transparent" HorizontalAlignment="Center" Margin="145,-0.5,146,0" VerticalAlignment="Top" Height="120" Click="btn_Lock" Grid.Row="1">
<Button.Background>
<ImageBrush Stretch="Uniform" ImageSource="Assets/safe_icon.png"/>
</Button.Background>
</Button>
And the ImageBox Items are normal items as well and have a blue background where the rest is transparent when clicking and white when selected. This should not be visible but transparent as well.
Thank you in advance

To make the change to the button control you need to change its style below is an example of changing the style of a button from the themebackgroundcolor to pink...
XAML Styles is a complicated subject ...
http://msdn.microsoft.com/en-us/expression/dd279541.aspx
I would recommend watching blend for developers from plural site as well to master this subject
In the below example the buttons color is changed from the default color to pink when it is clicked on...
<Page
x:Class="App5.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:App5"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Page.Resources>
<SolidColorBrush x:Key="PinkBrush" Color="#FFFF07A5"/>
<Style x:Key="ButtonStyle1" TargetType="Button">
<Setter Property="Background" Value="{ThemeResource ButtonBackgroundThemeBrush}"/>
<Setter Property="Foreground" Value="{ThemeResource ButtonForegroundThemeBrush}"/>
<Setter Property="BorderBrush" Value="{ThemeResource ButtonBorderThemeBrush}"/>
<Setter Property="BorderThickness" Value="{ThemeResource ButtonBorderThemeThickness}"/>
<Setter Property="Padding" Value="12,4,12,4"/>
<Setter Property="HorizontalAlignment" Value="Left"/>
<Setter Property="VerticalAlignment" Value="Center"/>
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
<Setter Property="FontWeight" Value="SemiBold"/>
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Border">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonPointerOverBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonPointerOverForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Border">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PinkBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonPressedForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Border">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonDisabledBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Border">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonDisabledBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonDisabledForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused">
<Storyboard>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="FocusVisualWhite"/>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="FocusVisualBlack"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Unfocused"/>
<VisualState x:Name="PointerFocused"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="Border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Margin="3">
<ContentPresenter x:Name="ContentPresenter" AutomationProperties.AccessibilityView="Raw" ContentTemplate="{TemplateBinding ContentTemplate}" ContentTransitions="{TemplateBinding ContentTransitions}" Content="{TemplateBinding Content}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
<Rectangle x:Name="FocusVisualWhite" IsHitTestVisible="False" Opacity="0" StrokeDashOffset="1.5" StrokeEndLineCap="Square" Stroke="{ThemeResource FocusVisualWhiteStrokeThemeBrush}" StrokeDashArray="1,1"/>
<Rectangle x:Name="FocusVisualBlack" IsHitTestVisible="False" Opacity="0" StrokeDashOffset="0.5" StrokeEndLineCap="Square" Stroke="{ThemeResource FocusVisualBlackStrokeThemeBrush}" StrokeDashArray="1,1"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Page.Resources>
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<Button Content="Button" HorizontalAlignment="Left" Height="118" Margin="291,228,0,0" VerticalAlignment="Top" Width="219" Style="{StaticResource ButtonStyle1}" />
</Grid>
</Page>

Transparent ComboBox in WinRT

So I'm trying to use the default ComboBox to display a list of choices. I want the background of the context menu that pops up when the combo box is selected to be Transparent. I can't find a property or style that does this and I haven't found much in the ways of doing this. Thanks in advance!

You need to extract a copy of the current template of your ComboBox with Visual Studio designer view or Blend and modify the Background property of the Border named "PopupBorder" to say Transparent. By default it's using {StaticResource ComboBoxPopupBackgroundThemeBrush}.
Here's a copy of the updated Style resource.
<Style
x:Key="ComboBoxStyle1"
TargetType="ComboBox">
<Setter
Property="Padding"
Value="8,0" />
<Setter
Property="Foreground"
Value="{StaticResource ComboBoxForegroundThemeBrush}" />
<Setter
Property="Background"
Value="{StaticResource ComboBoxBackgroundThemeBrush}" />
<Setter
Property="BorderBrush"
Value="{StaticResource ComboBoxBorderThemeBrush}" />
<Setter
Property="BorderThickness"
Value="{StaticResource ComboBoxBorderThemeThickness}" />
<Setter
Property="TabNavigation"
Value="Once" />
<Setter
Property="ScrollViewer.HorizontalScrollBarVisibility"
Value="Disabled" />
<Setter
Property="ScrollViewer.VerticalScrollBarVisibility"
Value="Auto" />
<Setter
Property="ScrollViewer.HorizontalScrollMode"
Value="Disabled" />
<Setter
Property="ScrollViewer.VerticalScrollMode"
Value="Auto" />
<Setter
Property="ScrollViewer.IsVerticalRailEnabled"
Value="True" />
<Setter
Property="ScrollViewer.IsDeferredScrollingEnabled"
Value="False" />
<Setter
Property="ScrollViewer.BringIntoViewOnFocusChange"
Value="True" />
<Setter
Property="HorizontalContentAlignment"
Value="Stretch" />
<Setter
Property="FontFamily"
Value="{StaticResource ContentControlThemeFontFamily}" />
<Setter
Property="FontSize"
Value="{StaticResource ControlContentThemeFontSize}" />
<Setter
Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<CarouselPanel />
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
<Setter
Property="Template">
<Setter.Value>
<ControlTemplate
TargetType="ComboBox">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition
Width="*" />
<ColumnDefinition
Width="32" />
</Grid.ColumnDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup
x:Name="CommonStates">
<VisualState
x:Name="Normal" />
<VisualState
x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Background"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPointerOverBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="BorderBrush"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPointerOverBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Fill"
Storyboard.TargetName="Highlight">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxSelectedPointerOverBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState
x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Background"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="BorderBrush"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<DoubleAnimation
Duration="0"
To="1"
Storyboard.TargetProperty="Opacity"
Storyboard.TargetName="PressedBackground" />
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="DropDownGlyph">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxArrowPressedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState
x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Background"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxDisabledBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="BorderBrush"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxDisabledBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="DropDownGlyph">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxArrowDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup
x:Name="FocusStates">
<VisualState
x:Name="Focused">
<Storyboard>
<DoubleAnimation
Duration="0"
To="1"
Storyboard.TargetProperty="Opacity"
Storyboard.TargetName="HighlightBackground" />
<DoubleAnimation
Duration="0"
To="1"
Storyboard.TargetProperty="Opacity"
Storyboard.TargetName="Highlight" />
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxFocusedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState
x:Name="FocusedPressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Fill"
Storyboard.TargetName="Highlight">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedHighlightThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState
x:Name="Unfocused" />
<VisualState
x:Name="PointerFocused" />
<VisualState
x:Name="FocusedDropDown">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Duration="0"
Storyboard.TargetProperty="Visibility"
Storyboard.TargetName="PopupBorder">
<DiscreteObjectKeyFrame
KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup
x:Name="DropDownStates">
<VisualState
x:Name="Opened">
<Storyboard>
<SplitOpenThemeAnimation
ClosedTargetName="ContentPresenter"
ContentTranslationOffset="0"
ContentTargetName="ScrollViewer"
ClosedLength="{Binding TemplateSettings.DropDownClosedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"
OffsetFromCenter="{Binding TemplateSettings.DropDownOffset, RelativeSource={RelativeSource Mode=TemplatedParent}}"
OpenedTargetName="PopupBorder"
OpenedLength="{Binding TemplateSettings.DropDownOpenedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}" />
</Storyboard>
</VisualState>
<VisualState
x:Name="Closed">
<Storyboard>
<SplitCloseThemeAnimation
ClosedTargetName="ContentPresenter"
ContentTranslationOffset="40"
ContentTranslationDirection="{Binding TemplateSettings.SelectedItemDirection, RelativeSource={RelativeSource Mode=TemplatedParent}}"
ContentTargetName="ScrollViewer"
ClosedLength="{Binding TemplateSettings.DropDownClosedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"
OffsetFromCenter="{Binding TemplateSettings.DropDownOffset, RelativeSource={RelativeSource Mode=TemplatedParent}}"
OpenedTargetName="PopupBorder"
OpenedLength="{Binding TemplateSettings.DropDownOpenedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}" />
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border
x:Name="Background"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Background="{TemplateBinding Background}"
Grid.ColumnSpan="2" />
<Rectangle
x:Name="PressedBackground"
Fill="{StaticResource ComboBoxPressedHighlightThemeBrush}"
Margin="{TemplateBinding BorderThickness}"
Opacity="0" />
<Border
x:Name="HighlightBackground"
BorderBrush="{StaticResource ComboBoxFocusedBorderThemeBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Background="{StaticResource ComboBoxFocusedBackgroundThemeBrush}"
Grid.ColumnSpan="2"
Opacity="0" />
<Rectangle
x:Name="Highlight"
Fill="{StaticResource ComboBoxSelectedBackgroundThemeBrush}"
Margin="{TemplateBinding BorderThickness}"
Opacity="0" />
<ContentPresenter
x:Name="ContentPresenter"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
Margin="{TemplateBinding Padding}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
<TextBlock
x:Name="DropDownGlyph"
Grid.Column="1"
Foreground="{StaticResource ComboBoxArrowForegroundThemeBrush}"
FontWeight="Bold"
FontSize="{StaticResource ComboBoxArrowThemeFontSize}"
FontFamily="{StaticResource SymbolThemeFontFamily}"
HorizontalAlignment="Right"
IsHitTestVisible="False"
Margin="0,0,6,4"
Text=""
VerticalAlignment="Center" />
<Popup
x:Name="Popup">
<Border
x:Name="PopupBorder"
BorderBrush="{StaticResource ComboBoxPopupBorderThemeBrush}"
BorderThickness="{StaticResource ComboBoxPopupBorderThemeThickness}"
Background="Transparent"
HorizontalAlignment="Stretch">
<ScrollViewer
x:Name="ScrollViewer"
BringIntoViewOnFocusChange="{TemplateBinding ScrollViewer.BringIntoViewOnFocusChange}"
Foreground="{StaticResource ComboBoxPopupForegroundThemeBrush}"
HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}"
VerticalSnapPointsType="OptionalSingle"
VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
VerticalSnapPointsAlignment="Near"
ZoomMode="Disabled">
<ItemsPresenter />
</ScrollViewer>
</Border>
</Popup>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Have you tried the following?
<ComboBox.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Background="Transparent" />
</ItemsPanelTemplate>
</ComboBox.ItemsPanel>
This works for me (without redefining the ControlTemplate).