Test

    <Style TargetType="{x:Type MenuItem}" x:Key="TopbarMenuItem" BasedOn="{StaticResource BaseStyle}">
        <Setter Property="WindowChrome.IsHitTestVisibleInChrome" Value="True"/>
        <Setter Property="TextOptions.TextFormattingMode" Value="Display"/>
        <Setter Property="Foreground" Value="{StaticResource WhiteBrush}"/>
        <Setter Property="HorizontalAlignment" Value="Center"/>
        <Setter Property="VerticalAlignment" Value="Center"/>
        <Setter Property="Margin" Value="0 6 0 6"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type MenuItem}" x:Name="rootMenuItem">
                    <Border x:Name="templateRoot">
                        <Grid VerticalAlignment="Center">
                            <ContentPresenter ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" ContentSource="Header" Margin="11 2 11 2"/>
                            <Popup x:Name="DropdownMenu" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" Placement="Bottom">
                                <Border x:Name="SubMenuBorder" Background="{StaticResource DropdownMenuBrush}" BorderBrush="{StaticResource BorderBrush}" BorderThickness="1">
                                    <Grid RenderOptions.ClearTypeHint="Enabled">
                                        <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" KeyboardNavigation.TabNavigation="Cycle"/>
                                    </Grid>
                                </Border>
                            </Popup>
                        </Grid>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsHighlighted" Value="True">
                            <Setter Property="Background" TargetName="templateRoot" Value="{StaticResource HoverBrush}"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    <Style TargetType="MenuItem" x:Key="MenuItemWithChildren">
        <Setter Property="ItemContainerStyleSelector" Value="{StaticResource ItemStyleSelector}"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type MenuItem}">
                    <Border x:Name="templateRoot" Background="{StaticResource DropdownMenuBrush}">
                        <Grid Margin="1">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="Auto"/>
                                <ColumnDefinition Width="Auto"/>
                                <ColumnDefinition Width="10*"/>
                                <ColumnDefinition Width="*"/>
                            </Grid.ColumnDefinitions>
                            <ContentPresenter x:Name="Icon" Grid.Column="0" Content="{TemplateBinding Icon}" ContentSource="Icon" VerticalAlignment="Center" HorizontalAlignment="Center" Margin="10 4 10 4" Height="16" Width="16"/>
                            <ContentPresenter Grid.Column="1" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" ContentSource="Header" VerticalAlignment="Center" HorizontalAlignment="Right" Margin="10 0 10 0"/>
                            <ContentPresenter Grid.Column="2" ContentTemplate="{TemplateBinding InputGestureText}" Content="{TemplateBinding InputGestureText}" ContentSource="InputGestureTextTemplate" TextBlock.Foreground="{StaticResource UnavailableTextBrush}" VerticalAlignment="Center" HorizontalAlignment="Right" Margin="10 0 10 0"/>
                            <Popup x:Name="DropdownMenu" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" Placement="Right">
                                <Border x:Name="SubMenuBorder" BorderBrush="{StaticResource BorderBrush}" BorderThickness="1">
                                    <Grid RenderOptions.ClearTypeHint="Enabled">
                                        <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" KeyboardNavigation.TabNavigation="Cycle"/>
                                    </Grid>
                                </Border>
                            </Popup>
                        </Grid>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="Icon" Value="{x:Null}">
                            <Setter Property="Visibility" TargetName="Icon" Value="Hidden"/>
                        </Trigger>
                        <Trigger Property="IsHighlighted" Value="True">
                            <Setter Property="Background" TargetName="templateRoot" Value="{StaticResource BlueBrush}"/>
                        </Trigger>
                        <Trigger Property="IsEnabled" Value="False">
                            <Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource UnavailableTextBrush}"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    <Style TargetType="MenuItem" x:Key="MenuItemWithoutChildren">
        <Setter Property="ItemContainerStyleSelector" Value="{StaticResource ItemStyleSelector}" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type MenuItem}">
                    <Border x:Name="templateRoot" Background="{StaticResource DropdownMenuBrush}">
                        <Grid Margin="1">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="Auto"/>
                                <ColumnDefinition Width="Auto"/>
                                <ColumnDefinition Width="10*"/>
                                <ColumnDefinition Width="*"/>
                            </Grid.ColumnDefinitions>
                            <ContentPresenter x:Name="Icon" Grid.Column="0" Content="{TemplateBinding Icon}" ContentSource="Icon" VerticalAlignment="Center" HorizontalAlignment="Center" Margin="10 4 10 4" Height="16" Width="16"/>
                            <ContentPresenter Grid.Column="1" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" ContentSource="Header" VerticalAlignment="Center" HorizontalAlignment="Right" Margin="10 0 10 0"/>
                            <ContentPresenter Grid.Column="2" ContentTemplate="{TemplateBinding InputGestureText}" Content="{TemplateBinding InputGestureText}" ContentSource="InputGestureTextTemplate" TextBlock.Foreground="{StaticResource UnavailableTextBrush}" VerticalAlignment="Center" HorizontalAlignment="Right" Margin="10 0 10 0"/>
                            <Path x:Name="RightArrow" Grid.Column="3" Data="M0 0L4 3.5 0 7z" Fill="{StaticResource WhiteBrush}" VerticalAlignment="Center" HorizontalAlignment="Right" Margin="10 0 10 0"/>
                            <Popup x:Name="DropdownMenu" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" Placement="Right">
                                <Border x:Name="SubMenuBorder" BorderBrush="{StaticResource BorderBrush}" BorderThickness="1">
                                    <Grid RenderOptions.ClearTypeHint="Enabled">
                                        <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" KeyboardNavigation.TabNavigation="Cycle"/>
                                    </Grid>
                                </Border>
                            </Popup>
                        </Grid>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="Icon" Value="{x:Null}">
                            <Setter Property="Visibility" TargetName="Icon" Value="Hidden"/>
                        </Trigger>
                        <Trigger Property="IsHighlighted" Value="True">
                            <Setter Property="Background" TargetName="templateRoot" Value="{StaticResource BlueBrush}"/>
                        </Trigger>
                        <Trigger Property="IsSubmenuOpen" Value="True">
                            <Setter Property="Fill" TargetName="RightArrow" Value="{StaticResource BlueBrush}"/>
                        </Trigger>
                        <Trigger Property="IsEnabled" Value="False">
                            <Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource UnavailableTextBrush}"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>