Shaker/Avalonia/SukiUI/Theme/ComboBoxItemStyle.axaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:icons="clr-namespace:SukiUI.Content">
    <Design.PreviewWith>
        <Border Padding="20">
            <StackPanel Spacing="10">
                <ComboBox PlaceholderText="Select an item">
                    <ComboBoxItem>Item 1</ComboBoxItem>
                    <ComboBoxItem>Item 2</ComboBoxItem>
                </ComboBox>
                <ComboBox Width="200"
                          HorizontalContentAlignment="Center"
                          IsEnabled="False"
                          SelectedIndex="1">
                    <ComboBoxItem>Item 1</ComboBoxItem>
                    <ComboBoxItem>Item 2</ComboBoxItem>
                </ComboBox>
            </StackPanel>
        </Border>
    </Design.PreviewWith>

    <Style Selector="ComboBoxItem">
        <Setter Property="Background" Value="Transparent" />
        <Setter Property="Padding" Value="2" />
        <Setter Property="HorizontalAlignment" Value="Stretch" />
        <Setter Property="HorizontalContentAlignment" Value="Left" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border Name="BorderBasicStyle"
                        Margin="3,0,3,3"
                        Padding="5,0"
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        CornerRadius="6">
                    <DockPanel>
                        <PathIcon Name="CheckSelected"
                                  Width="12"
                                  Height="12"
                                  Margin="0,0,3,0"
                                  Data="{x:Static icons:Icons.Check}"
                                  DockPanel.Dock="Right"
                                  Foreground="{DynamicResource SukiPrimaryColor}" />
                        <ContentPresenter Name="PART_ContentPresenter"
                                          Margin="1"
                                          Padding="5"
                                          HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                                          VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                                          Content="{TemplateBinding Content}"
                                          ContentTemplate="{TemplateBinding ContentTemplate}"
                                          CornerRadius="{DynamicResource SmallCornerRadius}" />

                    </DockPanel>
                </Border>
            </ControlTemplate>
        </Setter>
    </Style>

    <Style Selector="ComboBoxItem /template/ PathIcon#CheckSelected">
        <Setter Property="IsVisible" Value="False" />
    </Style>

    <Style Selector="ComboBoxItem:selected /template/ PathIcon#CheckSelected">
        <Setter Property="IsVisible" Value="True" />
    </Style>

    <Style Selector="ComboBoxItem:pointerover /template/ ContentPresenter">
        <Setter Property="Background" Value="Transparent" />
    </Style>

    <Style Selector="ComboBoxItem &gt; TextBlock">
        <Setter Property="Foreground" Value="{DynamicResource SukiText}" />
    </Style>

    <Style Selector="ComboBoxItem:selected /template/ ContentPresenter">
        <Setter Property="Background" Value="Transparent" />
    </Style>

    <Style Selector="ComboBoxItem:selected:focus /template/ ContentPresenter">
        <Setter Property="Background" Value="Transparent" />
    </Style>

    <Style Selector="ComboBoxItem:selected:pointerover /template/ ContentPresenter">
        <Setter Property="Background" Value="Transparent" />
    </Style>

    <Style Selector="ComboBoxItem:selected:focus:pointerover /template/ ContentPresenter">
        <Setter Property="Background" Value="Transparent" />
    </Style>

    <Style Selector="ComboBoxItem:pointerover /template/ Border#BorderBasicStyle">
        <Setter Property="Background" Value="{DynamicResource SukiLightBorderBrush}" />
    </Style>

    <Style Selector="ComboBoxItem:selected /template/ Border#BorderBasicStyle">
        <Setter Property="Background" Value="{DynamicResource SukiLightBorderBrush}" />
    </Style>

    <Style Selector="ComboBoxItem:selected:focus /template/ Border#BorderBasicStyle">
        <Setter Property="Background" Value="{DynamicResource SukiLightBorderBrush}" />
    </Style>

    <Style Selector="ComboBoxItem:selected:pointerover /template/ Border#BorderBasicStyle">
        <Setter Property="Background" Value="{DynamicResource SukiLightBorderBrush}" />
    </Style>

    <Style Selector="ComboBoxItem:selected:focus:pointerover /template/ Border#BorderBasicStyle">
        <Setter Property="Background" Value="{DynamicResource SukiLightBorderBrush}" />
    </Style>
</Styles>