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

<Style xmlns="https://github.com/avaloniaui"
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
       Selector=":is(UserControl)"
       x:ClassModifier="internal">
  <Style.Resources>
    <ControlTheme x:Key="FluentUserControl" TargetType="UserControl">
      <Setter Property="Template">
        <ControlTemplate>
          <ContentPresenter Name="PART_ContentPresenter"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            CornerRadius="{TemplateBinding CornerRadius}"
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            Content="{TemplateBinding Content}"
                            Padding="{TemplateBinding Padding}"
                            VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                            HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"/>
        </ControlTemplate>
      </Setter>
    </ControlTheme>
  </Style.Resources>
  
  <Setter Property="Theme" Value="{StaticResource FluentUserControl}" />
</Style>