aistudio-wpf-diagram/Others/Fluent.Ribbon/Fluent.Ribbon/Themes/Controls/ComboBoxItem.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:Fluent="clr-namespace:Fluent">
    <ControlTemplate TargetType="{x:Type ComboBoxItem}"
                     x:Key="ComboBoxItemTemplate">
        <Grid Background="Transparent">
            <Grid Margin="0 0.5"
                  Background="{TemplateBinding Background}">
                <Border x:Name="ItemBackground"
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        IsHitTestVisible="False"
                        SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                <ContentPresenter x:Name="contentPresenter"
                                  Margin="{TemplateBinding Padding}"
                                  HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                  VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                  TextElement.Foreground="{TemplateBinding Foreground}" />
            </Grid>
        </Grid>
        <ControlTemplate.Triggers>
            <Trigger Property="IsSelected" Value="True">
                <Setter TargetName="ItemBackground" Property="Background" Value="{DynamicResource Fluent.Ribbon.Brushes.Button.Pressed.Background}" />
            </Trigger>
            <Trigger Property="IsMouseOver" Value="True">
                <Setter TargetName="ItemBackground" Property="Background" Value="{DynamicResource Fluent.Ribbon.Brushes.Button.MouseOver.Background}" />
            </Trigger>
            <Trigger Property="IsEnabled" Value="False">
                <Setter TargetName="contentPresenter" Property="Opacity" Value="0.5" />
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

    <Style x:Key="ComboBoxItemStyle"
           TargetType="{x:Type ComboBoxItem}">
        <Setter Property="Padding"
                Value="3" />
        <Setter Property="Template"
                Value="{DynamicResource ComboBoxItemTemplate}" />
    </Style>
</ResourceDictionary>