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

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <ControlTheme x:Key="{x:Type ItemsControl}"
                TargetType="ItemsControl">
    <Setter Property="Template">
      <ControlTemplate>
        <Border Padding="{TemplateBinding Padding}"
                Background="{TemplateBinding Background}"
                BorderBrush="{TemplateBinding BorderBrush}"
                BorderThickness="{TemplateBinding BorderThickness}"
                CornerRadius="{TemplateBinding CornerRadius}">
          <ItemsPresenter Name="PART_ItemsPresenter"
                          ItemTemplate="{TemplateBinding ItemTemplate}"
                          Items="{TemplateBinding Items}"
                          ItemsPanel="{TemplateBinding ItemsPanel}" />
        </Border>
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>