Nitrox/Nitrox.Launcher/Models/Styles/Theme/ComboBoxStyle.axaml

<Styles
    xmlns="https://github.com/avaloniaui"
    xmlns:design="clr-namespace:Nitrox.Launcher.Models.Design"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Design.PreviewWith>
        <Panel Background="CornflowerBlue">
            <StackPanel
                Margin="10"
                Orientation="Horizontal"
                Spacing="20">
                <StackPanel Spacing="10">
                    <Border
                        Background="White"
                        HorizontalAlignment="Center"
                        Padding="10">
                        <ThemeVariantScope RequestedThemeVariant="Light">
                            <Border Margin="0,0,0,110">
                                <ComboBox
                                    HorizontalAlignment="Stretch"
                                    PlaceholderText="Light"
                                    Width="200">
                                    <ComboBoxItem Content="Item 1" />
                                    <ComboBoxItem Content="Item 2" />
                                    <ComboBoxItem Content="Item 3" />
                                </ComboBox>
                            </Border>
                        </ThemeVariantScope>
                    </Border>
                    <Border
                        Background="Black"
                        HorizontalAlignment="Center"
                        Padding="10">
                        <ThemeVariantScope RequestedThemeVariant="Dark">
                            <Border Margin="0,0,0,110">
                                <ComboBox
                                    HorizontalAlignment="Stretch"
                                    PlaceholderText="Dark"
                                    Width="200">
                                    <ComboBoxItem Content="Item 1" />
                                    <ComboBoxItem Content="Item 2" />
                                    <ComboBoxItem Content="Item 3" />
                                </ComboBox>
                            </Border>
                        </ThemeVariantScope>
                    </Border>
                </StackPanel>
            </StackPanel>
        </Panel>
    </Design.PreviewWith>

    <Style Selector="ComboBox">
        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
        <Setter Property="VerticalContentAlignment" Value="Center" />
        <Setter Property="Padding" Value="14" />
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Disabled" />
        <Setter Property="BorderThickness" Value="0" />
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
        <Setter Property="ScrollViewer.IsScrollInertiaEnabled" Value="true" />
        <Setter Property="FontSize" Value="16" />
        <Setter Property="FontWeight" Value="Normal" />
        <Setter Property="CornerRadius" Value="8" />
        <Setter Property="MinWidth" Value="100" />
        <Setter Property="MinHeight" Value="30" />
        <Setter Property="Background" Value="{DynamicResource BrandControlBackground}" />
        <Style Selector="^ /template/ ContentPresenter">
            <Style Selector="^ TextBlock">
                <Setter Property="Foreground" Value="{DynamicResource BrandBlack}" />
            </Style>
            <Style Selector="^ TextBlock#PlaceholderTextBlock">
                <Setter Property="Foreground" Value="{DynamicResource BrandSubText}" />
            </Style>
            <Style Selector="^ PathIcon">
                <Setter Property="Foreground" Value="{DynamicResource BrandBlack}" />
            </Style>
        </Style>

        <Style Selector="^:pointerover /template/ Border#Background">
            <Setter Property="Cursor" Value="Hand" />
            <Setter Property="Background" Value="{DynamicResource BrandControlBackground}" />
        </Style>

        <Setter Property="Transitions">
            <Transitions>
                <TransformOperationsTransition Duration="0:0:.075" Property="RenderTransform" />
                <DoubleTransition Duration="0:0:0.15" Property="Opacity" />
            </Transitions>
        </Setter>

        <Setter Property="Template">
            <ControlTemplate>
                <DataValidationErrors>
                    <Grid>
                        <Grid ColumnDefinitions="*,32" IsHitTestVisible="True">
                            <Border
                                Background="{TemplateBinding Background}"
                                BorderBrush="{TemplateBinding BorderBrush}"
                                BorderThickness="{TemplateBinding BorderThickness}"
                                CornerRadius="{TemplateBinding CornerRadius}"
                                Grid.ColumnSpan="2"
                                MinWidth="{DynamicResource ComboBoxThemeMinWidth}"
                                x:Name="Background" />
                            <TextBlock
                                Grid.Column="0"
                                HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                IsVisible="{TemplateBinding SelectionBoxItem,
                                                            Converter={x:Static ObjectConverters.IsNull}}"
                                Margin="{TemplateBinding Padding}"
                                Text="{TemplateBinding PlaceholderText}"
                                VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                x:Name="PlaceholderTextBlock" />
                            <ContentControl
                                Content="{TemplateBinding SelectionBoxItem}"
                                ContentTemplate="{TemplateBinding ItemTemplate}"
                                Grid.Column="0"
                                HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                                Margin="{TemplateBinding Padding}"
                                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                                x:Name="ContentPresenter" />
                            <Border
                                Background="Transparent"
                                Grid.Column="1"
                                HorizontalAlignment="Right"
                                IsVisible="False"
                                Margin="0,1,1,1"
                                Width="30"
                                x:Name="DropDownOverlay" />
                            <PathIcon
                                Data="M1939 486L2029 576L1024 1581L19 576L109 486L1024 1401L1939 486Z"
                                Foreground="{TemplateBinding Foreground}"
                                Grid.Column="1"
                                Height="12"
                                HorizontalAlignment="Right"
                                IsHitTestVisible="False"
                                Margin="0,0,10,0"
                                UseLayoutRounding="False"
                                VerticalAlignment="Center"
                                Width="12"
                                x:Name="DropDownGlyph" />
                        </Grid>

                        <Popup
                            Grid.Column="0"
                            InheritsTransform="True"
                            IsLightDismissEnabled="True"
                            IsOpen="{TemplateBinding IsDropDownOpen,
                                                     Mode=TwoWay}"
                            MaxHeight="{TemplateBinding MaxDropDownHeight}"
                            MinWidth="{Binding Bounds.Width, RelativeSource={RelativeSource TemplatedParent}}"
                            Name="PART_Popup"
                            PlacementTarget="Background"
                            WindowManagerAddShadowHint="False">
                            <Border
                                Background="{TemplateBinding Background}"
                                BorderBrush="{DynamicResource BrandWhite}"
                                BorderThickness="1,0,2,2"
                                CornerRadius="{DynamicResource OverlayCornerRadius}"
                                HorizontalAlignment="Stretch"
                                Padding="{DynamicResource ComboBoxDropdownBorderPadding}"
                                x:Name="PopupBorder">
                                <ScrollViewer HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
                                    <ItemsPresenter
                                        ItemsPanel="{TemplateBinding ItemsPanel}"
                                        Margin="{DynamicResource ComboBoxDropdownContentMargin}"
                                        Name="PART_ItemsPresenter" />
                                </ScrollViewer>
                            </Border>
                        </Popup>

                    </Grid>
                </DataValidationErrors>
            </ControlTemplate>
        </Setter>

        <Style Selector="^:pointerover">
            <Setter Property="Opacity" Value=".75" />
        </Style>

        <Style Selector="^:pressed">
            <Setter Property="RenderTransform" Value="scale(0.98)" />
        </Style>

        <Style Selector="^:disabled">
            <Setter Property="Background" Value="{TemplateBinding Background}" />
        </Style>
    </Style>
</Styles>