Avalonia/samples/XamlTestApplicationPcl/Window.paml

<Style xmlns="https://github.com/perspex" Selector="Window">
  <Setter Property="FontFamily" Value="Segoe UI"/>
  <Setter Property="FontSize" Value="12"/>
  <Setter Property="Template">
    <ControlTemplate>
      <Border Background="{TemplateBinding Background}">
        <AdornerDecorator>
          <ContentPresenter Content="{TemplateBinding Content}" Margin="{TemplateBinding Padding}"/>
        </AdornerDecorator>
      </Border>
    </ControlTemplate>
  </Setter>
</Style>