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