Avalonia/src/Avalonia.Themes.Fluent/Controls/Window.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">
  <ControlTheme x:Key="{x:Type Window}" TargetType="Window">
    <Setter Property="Background" Value="{DynamicResource SystemRegionBrush}"/>
    <Setter Property="TransparencyBackgroundFallback" Value="{DynamicResource SystemControlBackgroundAltHighBrush}" />
    <Setter Property="TopLevel.SystemBarColor" Value="{DynamicResource SystemControlBackgroundAltHighBrush}"/>
    <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>
    <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}"/>
    <Setter Property="FontFamily" Value="{DynamicResource ContentControlThemeFontFamily}" />
    <Setter Property="Template">
      <ControlTemplate>
        <Panel>
          <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
          <Border Background="{TemplateBinding Background}" IsHitTestVisible="False" />
          <Panel Background="Transparent" Margin="{TemplateBinding WindowDecorationMargin}" />
          <VisualLayerManager Name="PART_VisualLayerManager">
            <ContentPresenter Name="PART_ContentPresenter"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Content="{TemplateBinding Content}"
                              Margin="{TemplateBinding Padding}"
                              HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                              VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
          </VisualLayerManager>
        </Panel>
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>