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

<Style xmlns="https://github.com/avaloniaui"
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
       Selector="Menu">
  <Design.PreviewWith>
    <Border Padding="20">
      <Menu>
        <MenuItem Header="New" />
        <MenuItem Header="Open" />
      </Menu>
    </Border>
  </Design.PreviewWith>

  <Style.Resources>
    <x:Double x:Key="MenuBarHeight">32</x:Double>
  </Style.Resources>

  <Setter Property="Background" Value="Transparent" />
  <Setter Property="Height" Value="{DynamicResource MenuBarHeight}" />
  <Setter Property="Template">
    <ControlTemplate>
      <Border Background="{TemplateBinding Background}"
              BorderBrush="{TemplateBinding BorderBrush}"
              BorderThickness="{TemplateBinding BorderThickness}"
              HorizontalAlignment="Stretch"
              Padding="{TemplateBinding Padding}">
        <ItemsPresenter Name="PART_ItemsPresenter"
                        Items="{TemplateBinding Items}"
                        ItemsPanel="{TemplateBinding ItemsPanel}"
                        ItemTemplate="{TemplateBinding ItemTemplate}"
                        VerticalAlignment="Stretch"
                        KeyboardNavigation.TabNavigation="Continue" />
      </Border>
    </ControlTemplate>
  </Setter>
</Style>