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I can't figure out how to set the Highlight brush of a Combobox (the color that will mark the item being selected on mouse over) to a certain Brush. I have used Edit Template -> Copy to get copy of the template where I try to set the SystemColors.HighlightBrushKey. I have seen answers that one should be able to set the brush by defining it like this in the Style resource, but it just does not work.

I have also tried to set the Grid's Style.Resource in which the comobox is placed, that did not help either.

Please see line 3 and line 15 where I set the color to red. No effect. What am I doing wrong?

 <Style x:Key="TeachpendantVMVisionServerComboBoxStyle" TargetType="x:Type ComboBox">
 <Style.Resources>
 <SolidColorBrush x:Key="x:Static SystemColors.HighlightBrushKey" Color="Red" />
 <SolidColorBrush x:Key="x:Static SystemColors.ControlBrushKey" Color="Red" />
 </Style.Resources>
 <Setter Property="HorizontalContentAlignment" Value="Center"/>
 <Setter Property="VerticalContentAlignment" Value="Center"/>
 <Setter Property="Template">
 <Setter.Value>
 <ControlTemplate TargetType="x:Type ComboBox">
 <Grid x:Name="grid">
 <Grid.Style>
 <Style>
 <Style.Resources>
 <SolidColorBrush x:Key="x:Static SystemColors.HighlightBrushKey" Color="Red" />
 <SolidColorBrush x:Key="x:Static SystemColors.ControlBrushKey" Color="Red" />
 </Style.Resources>
 </Style>
 </Grid.Style>

 <Grid.ColumnDefinitions>
 <ColumnDefinition/>
 <ColumnDefinition MaxWidth="18"/>
 </Grid.ColumnDefinitions>
 <VisualStateManager.VisualStateGroups>
 <VisualStateGroup x:Name="ValidationStates">
 <VisualState x:Name="Valid"/>
 <VisualState x:Name="InvalidFocused">
 <Storyboard>
 <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(Border.BorderBrush)" Storyboard.TargetName="PART_EditableTextBox">
 <DiscreteObjectKeyFrame KeyTime="0">
 <DiscreteObjectKeyFrame.Value>
 <SolidColorBrush Color="Red"/>
 </DiscreteObjectKeyFrame.Value>
 </DiscreteObjectKeyFrame>
 </ObjectAnimationUsingKeyFrames>
 <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill)" Storyboard.TargetName="path">
 <DiscreteObjectKeyFrame KeyTime="0" Value="StaticResource MoveTextBox.Invalid.BorderBrush"/>
 </ObjectAnimationUsingKeyFrames>
 <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Border.BorderBrush).(SolidColorBrush.Color)" Storyboard.TargetName="toggleButton">
 <EasingColorKeyFrame KeyTime="0" Value="#FFFB0000"/>
 </ColorAnimationUsingKeyFrames>
 </Storyboard>
 </VisualState>
 <VisualState x:Name="InvalidUnfocused">
 <Storyboard>
 <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(Border.BorderBrush)" Storyboard.TargetName="PART_EditableTextBox">
 <DiscreteObjectKeyFrame KeyTime="0" Value="StaticResource MoveTextBox.Invalid.BorderBrush"/>
 </ObjectAnimationUsingKeyFrames>
 <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Border.BorderBrush).(SolidColorBrush.Color)" Storyboard.TargetName="toggleButton">
 <EasingColorKeyFrame KeyTime="0" Value="Red"/>
 </ColorAnimationUsingKeyFrames>
 <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill)" Storyboard.TargetName="path">
 <DiscreteObjectKeyFrame KeyTime="0">
 <DiscreteObjectKeyFrame.Value>
 <SolidColorBrush Color="Red"/>
 </DiscreteObjectKeyFrame.Value>
 </DiscreteObjectKeyFrame>
 </ObjectAnimationUsingKeyFrames>
 </Storyboard>
 </VisualState>
 </VisualStateGroup>
 </VisualStateManager.VisualStateGroups>
 <TextBox x:Name="PART_EditableTextBox" 
 Padding="5,0,0,0" 
 Height="TemplateBinding Height">
 <TextBox.Template>
 <ControlTemplate TargetType="x:Type TextBox">
 <Grid>
 <Border 
 x:Name="border"
 CornerRadius="3,0,0,3" 
 BorderThickness="1,1,0,1"
 Background="DynamicResource Button.Static.Background"
 BorderBrush="DynamicResource Button.Static.Background">
 <ScrollViewer x:Name="PART_ContentHost"/>
 </Border>
 </Grid>
 </ControlTemplate>
 </TextBox.Template>
 </TextBox>
 <ToggleButton x:Name="toggleButton" Grid.Column="1" Margin="0" 
 Height="TemplateBinding Height" 
 Focusable="False"
 IsChecked="Binding IsDropDownOpen, Mode=TwoWay, RelativeSource=RelativeSource TemplatedParent"
 ClickMode="Press" BorderBrush="#FFABADB3">
 <ToggleButton.Template>
 <ControlTemplate TargetType="x:Type ToggleButton">
 <Border 
 Background="DynamicResource Button.Checked.Background" 
 x:Name="border" 
 CornerRadius="0,3,3,0" 
 BorderThickness="0,1,1,1"
 BorderBrush="DynamicResource Button.Static.Background">
 <ContentPresenter />
 </Border>
 </ControlTemplate>
 </ToggleButton.Template>
 <Path x:Name="path" Grid.Column="1"
 HorizontalAlignment="Center"
 VerticalAlignment="Center"
 Data="M 0 0 L 4 4 L 8 0 Z"
 Fill="Gold" />
 </ToggleButton>
 <ContentPresenter x:Name="ContentSite"
 Content="TemplateBinding SelectionBoxItem"
 ContentTemplate="TemplateBinding SelectionBoxItemTemplate"
 ContentTemplateSelector="TemplateBinding ItemTemplateSelector"
 VerticalAlignment="Center"
 HorizontalAlignment="Right"
 Margin="5,0,0,0" RecognizesAccessKey="True"/>
 <Popup x:Name="Popup"
 Placement="Bottom"
 IsOpen="TemplateBinding IsDropDownOpen"
 AllowsTransparency="True" 
 Focusable="True"
 PopupAnimation="Slide" 
 OverridesDefaultStyle="True">
 <Grid x:Name="DropDown"
 SnapsToDevicePixels="True" 
 MinWidth="TemplateBinding ActualWidth"
 MaxHeight="TemplateBinding MaxDropDownHeight"> 
 <Border 
 x:Name="DropDownBorder"
 BorderThickness="1"
 CornerRadius="5"
 Background="Azure"
 BorderBrush="DarkGray"> 
 </Border> 
 <ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True"> 
 <StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
 </ScrollViewer>
 </Grid>
 </Popup>
 </Grid>
 </ControlTemplate>
 </Setter.Value>
 </Setter>
 <Setter Property="Validation.ErrorTemplate">
 <Setter.Value>
 <ControlTemplate/>
 </Setter.Value>
 </Setter>
 </Style>

It was fairly simple after all (but not obvious where to find it).
Just edit a copy of the ItemsContainerStyle and all these Brushes can be set from there.

To Change the layout of the ComboBox (creating a round combobox or changing colors)
Style="DynamicResource MyComboBoxStyle"

To Change the hoover and selected Highlight colors

ItemContainerStyle="DynamicResource MyComboBoxItemS"

The complete Combobox would look like this then in XAML

<ComboBox x:Name="positionsComboBox"
 Grid.Column="1" 
 Grid.ColumnSpan="3"
 IsReadOnly = "True"
 IsEditable = "False" 
 Margin="3,0,3,0"
 ItemsSource="Binding IDs, Source=StaticResource Locator, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay" 
 SelectedValue="Binding SelectedID, Source=StaticResource Locator, Mode=TwoWay" 
 Style="DynamicResource MyComboBoxStyle"
 IsSynchronizedWithCurrentItem="True" 
 ItemContainerStyle="DynamicResource MyComboBoxItemContainerStyle" 
 FontWeight="Bold" 
 FontSize="24"/>