Avalonia/src/Avalonia.Themes.Default/ContentControl.xaml

<Style xmlns="https://github.com/avaloniaui" Selector="ContentControl">
  <Setter Property="Template">
    <ControlTemplate>
      <ContentPresenter Name="PART_ContentPresenter"
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        ContentTemplate="{TemplateBinding ContentTemplate}"
                        Content="{TemplateBinding Content}"
                        Padding="{TemplateBinding Padding}"
                        VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                        HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"/>
    </ControlTemplate>
  </Setter>
</Style>