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

<Style xmlns="https://github.com/avaloniaui"
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
       Selector="PopupRoot">
  <Setter Property="Background" Value="{x:Null}"/>
  <Setter Property="TransparencyLevelHint" Value="Transparent" />
  <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>
  <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
  <Setter Property="FontFamily" Value="{x:Static FontFamily.Default}" />
  <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>
</Style>