HandyControl/src/Avalonia/HandyControl_Avalonia/Themes/Styles/Menu.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:hc="https://handyorg.github.io/handycontrol"
                    xmlns:sys="using:System">

    <ControlTheme x:Key="MenuItemBaseStyle"
                  TargetType="MenuItem">
        <Setter Property="Padding"
                Value="{Binding $self.(hc:MenuAttach.ItemPadding)}" />
        <Setter Property="MinHeight"
                Value="{Binding $self.(hc:MenuAttach.ItemMinHeight)}" />
        <Setter Property="Background"
                Value="Transparent" />
        <Setter Property="BorderThickness"
                Value="0" />
        <Setter Property="Foreground"
                Value="{DynamicResource PrimaryTextBrush}" />
        <Setter Property="Margin"
                Value="0,0,0,2" />
        <Setter Property="CornerRadius"
                Value="{Binding $self.(hc:BorderElement.CornerRadius)}" />
        <Setter Property="MinWidth"
                Value="240" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border Name="root"
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        CornerRadius="{TemplateBinding CornerRadius}">
                    <Grid Margin="{TemplateBinding Padding}">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="26" />
                            <ColumnDefinition Width="*" />
                            <ColumnDefinition Width="30" />
                            <ColumnDefinition Width="Auto"
                                              SharedSizeGroup="MenuItemIGT" />
                            <ColumnDefinition Width="20" />
                        </Grid.ColumnDefinitions>
                        <ContentControl Grid.Column="0"
                                        x:Name="PART_ToggleIconPresenter"
                                        IsVisible="False"
                                        Width="16"
                                        Height="16"
                                        HorizontalAlignment="Right"
                                        VerticalAlignment="Center" />
                        <ContentControl Grid.Column="0"
                                        Name="PART_IconPresenter"
                                        Width="16"
                                        Height="16"
                                        HorizontalAlignment="Right"
                                        VerticalAlignment="Center"
                                        Content="{TemplateBinding Icon}" />
                        <ContentPresenter Name="PART_HeaderPresenter"
                                          Grid.Column="1"
                                          VerticalAlignment="Center"
                                          Content="{TemplateBinding Header}"
                                          ContentTemplate="{TemplateBinding HeaderTemplate}">
                            <ContentPresenter.DataTemplates>
                                <DataTemplate DataType="sys:String">
                                    <AccessText Text="{Binding}" />
                                </DataTemplate>
                            </ContentPresenter.DataTemplates>
                        </ContentPresenter>
                        <TextBlock x:Name="PART_InputGestureText"
                                   Grid.Column="3"
                                   VerticalAlignment="Center"
                                   HorizontalAlignment="Left"
                                   Text="{TemplateBinding InputGesture, Converter={StaticResource PlatformKeyGestureConverter}}" />
                        <Path Name="rightArrow"
                              Grid.Column="4"
                              Margin="0,0,10,0"
                              Width="10"
                              Height="10"
                              HorizontalAlignment="Left"
                              VerticalAlignment="Center"
                              Stretch="Uniform"
                              Data="{StaticResource RightGeometry}"
                              Fill="{TemplateBinding Foreground}" />
                        <Popup Grid.Column="1"
                               Name="PART_Popup"
                               IsLightDismissEnabled="False"
                               IsOpen="{TemplateBinding IsSubMenuOpen, Mode=TwoWay}"
                               Placement="RightEdgeAlignedTop"
                               VerticalOffset="{Binding $self.(hc:MenuAttach.PopupVerticalOffset)}"
                               HorizontalOffset="{Binding $self.(hc:MenuAttach.PopupHorizontalOffset)}">
                            <Border Background="{DynamicResource RegionBrush}"
                                    BorderBrush="{DynamicResource BorderBrush}"
                                    BorderThickness="1"
                                    Margin="8"
                                    Effect="{StaticResource EffectShadow2}"
                                    CornerRadius="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}}">
                                <ScrollViewer Theme="{StaticResource ScrollViewerUpDown}"
                                              Margin="2,2,2,0">
                                    <ItemsPresenter Name="PART_ItemsPresenter"
                                                    Grid.IsSharedSizeScope="True"
                                                    ItemsPanel="{TemplateBinding ItemsPanel}" />
                                </ScrollViewer>
                            </Border>
                        </Popup>
                    </Grid>
                </Border>
            </ControlTemplate>
        </Setter>

        <Style Selector="^:selected /template/ Border#root">
            <Setter Property="Background"
                    Value="{DynamicResource SecondaryRegionBrush}" />
        </Style>

        <Style Selector="^:empty /template/ Path#rightArrow">
            <Setter Property="IsVisible"
                    Value="False" />
        </Style>
    </ControlTheme>

    <ControlTheme x:Key="MenuBaseStyle"
                  TargetType="Menu">
        <Setter Property="hc:MenuAttach.ItemPadding"
                Value="{StaticResource DefaultControlPadding}" />
        <Setter Property="hc:MenuAttach.ItemMinHeight"
                Value="{StaticResource DefaultControlHeight}" />
        <Setter Property="hc:MenuAttach.PopupVerticalOffset"
                Value="-16" />
        <Setter Property="hc:MenuAttach.PopupHorizontalOffset"
                Value="1" />
        <Setter Property="Background"
                Value="{DynamicResource RegionBrush}" />
        <Setter Property="Foreground"
                Value="{DynamicResource PrimaryTextBrush}" />
        <Setter Property="hc:BorderElement.CornerRadius"
                Value="{StaticResource DefaultCornerRadius}" />
        <Setter Property="CornerRadius"
                Value="{Binding $self.(hc:BorderElement.CornerRadius)}" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border Padding="{TemplateBinding Padding}"
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        CornerRadius="{TemplateBinding CornerRadius}">
                    <ItemsPresenter Name="PART_ItemsPresenter"
                                    ItemsPanel="{TemplateBinding ItemsPanel}"
                                    KeyboardNavigation.TabNavigation="Continue" />
                </Border>
            </ControlTemplate>
        </Setter>
    </ControlTheme>

    <ControlTheme x:Key="{x:Type MenuItem}"
                  BasedOn="{StaticResource MenuItemBaseStyle}"
                  TargetType="MenuItem" />

    <ControlTheme x:Key="{x:Type Menu}"
                  BasedOn="{StaticResource MenuBaseStyle}"
                  TargetType="Menu" />

    <ControlTheme x:Key="Menu.Small"
                  BasedOn="{StaticResource MenuBaseStyle}"
                  TargetType="Menu">
        <Setter Property="hc:MenuAttach.ItemPadding"
                Value="6,0" />
        <Setter Property="hc:MenuAttach.ItemMinHeight"
                Value="20" />
        <Setter Property="hc:MenuAttach.PopupVerticalOffset"
                Value="-11" />
        <Setter Property="hc:MenuAttach.PopupHorizontalOffset"
                Value="-2" />
    </ControlTheme>

</ResourceDictionary>