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

<Style xmlns="https://github.com/avaloniaui" Selector="EmbeddableControlRoot">
  <Setter Property="Background" Value="{DynamicResource SystemControlBackgroundAltHighBrush}"/>
  <Setter Property="FontSize" Value="{DynamicResource ContentControlFontSize}"/>
  <Setter Property="Template">
    <ControlTemplate>
      <Panel>
        <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
        <Border Background="{TemplateBinding Background}">
          <VisualLayerManager>
            <ContentPresenter Name="PART_ContentPresenter"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Content="{TemplateBinding Content}"
                              Margin="{TemplateBinding Padding}"/>
          </VisualLayerManager>
        </Border>
      </Panel>
    </ControlTemplate>
  </Setter>
</Style>