Avalonia/src/Avalonia.Themes.Simple/Controls/NativeMenuBar.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:default="using:Avalonia.Themes.Simple"
                    x:ClassModifier="internal">
  <default:IBitmapToImageConverter x:Key="AvaloniaThemesSimpleNativeMenuBarIBitmapToImageConverter" />
  <ControlTheme x:Key="{x:Type NativeMenuBar}"
                TargetType="NativeMenuBar">
    <Setter Property="Template">
      <ControlTemplate>
        <Menu IsVisible="{Binding !$parent[TopLevel].(NativeMenu.IsNativeMenuExported)}"
              ItemsSource="{Binding $parent[TopLevel].(NativeMenu.Menu).Items}">
          <Menu.Styles>
            <Style Selector="MenuItem" x:DataType="NativeMenuItem">
              <Setter Property="Header" Value="{Binding Header}"/>
              <Setter Property="IsEnabled" Value="{Binding IsEnabled}"/>
              <Setter Property="InputGesture" Value="{Binding Gesture}"/>
              <Setter Property="ItemsSource" Value="{Binding Menu.Items}"/>
              <Setter Property="Command" Value="{Binding Command}"/>
              <Setter Property="CommandParameter" Value="{Binding CommandParameter}"/>
              <Setter Property="(NativeMenuBar.EnableMenuItemClickForwarding)" Value="True"/>
              <!--NativeMenuItem is IBitmap and MenuItem is Image-->
              <Setter Property="Icon" Value="{Binding Icon , Converter={StaticResource AvaloniaThemesSimpleNativeMenuBarIBitmapToImageConverter}}"/>
            </Style>
          </Menu.Styles>
        </Menu>
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>