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

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <ControlTheme x:Key="{x:Type PopupRoot}" TargetType="PopupRoot">
    <Setter Property="Background" Value="{x:Null}" />
    <Setter Property="TransparencyLevelHint" Value="Transparent" />
    <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}" />
    <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}" />
    <Setter Property="FontFamily" Value="{DynamicResource ContentControlThemeFontFamily}" />
    <Setter Property="FontWeight" Value="400" />
    <Setter Property="FontStyle" Value="Normal" />
    <Setter Property="Template">
      <ControlTemplate>
        <LayoutTransformControl LayoutTransform="{TemplateBinding Transform}">
          <Panel>
            <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
            <VisualLayerManager IsPopup="True">
              <ContentPresenter Name="PART_ContentPresenter"
                                Background="{TemplateBinding Background}"
                                ContentTemplate="{TemplateBinding ContentTemplate}"
                                Content="{TemplateBinding Content}"
                                Padding="{TemplateBinding Padding}" />
            </VisualLayerManager>
          </Panel>
        </LayoutTransformControl>
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>