Avalonia/src/Avalonia.Themes.Default/EmbeddableControlRoot.xaml

<Style xmlns="https://github.com/avaloniaui" Selector="EmbeddableControlRoot">
  <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
  <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
  <Setter Property="Template">
    <ControlTemplate>
      <Border Background="{TemplateBinding Background}">
        <AdornerDecorator>
          <ContentPresenter Name="PART_ContentPresenter" 
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            Content="{TemplateBinding Content}" 
                            Margin="{TemplateBinding Padding}"/>
        </AdornerDecorator>
      </Border>
    </ControlTemplate>
  </Setter>
</Style>