menuitem_transparent

//in app.xaml
//...
<ControlTemplate x:Key="{x:Static MenuItem.TopLevelHeaderTemplateKey}" TargetType="{x:Type MenuItem}">
            <Border Name="Border" >
                <Grid>
                    <ContentPresenter
        Margin="6,3,6,3"
        ContentSource="Header"
        RecognizesAccessKey="True" />
                    <Popup
      Name="Popup"
      Placement="Bottom"
      IsOpen="{TemplateBinding IsSubmenuOpen}"
      AllowsTransparency="True"
      Focusable="False"
      PopupAnimation="Fade">
                        <Border
        Name="SubmenuBorder"
        SnapsToDevicePixels="True"

         Background= "#0D000000"   >
                            <StackPanel
          IsItemsHost="True"
          KeyboardNavigation.DirectionalNavigation="Cycle" />
                        </Border>
                    </Popup>
                </Grid>
            </Border>
            <ControlTemplate.Triggers>
                <Trigger Property="IsSuspendingPopupAnimation" Value="true">
                    <Setter TargetName="Popup" Property="PopupAnimation" Value="None"/>
                </Trigger>
                <Trigger Property="IsHighlighted" Value="true">
                    <Setter TargetName="Border" Property="Background" Value="#C0C0C0"/>
                    <Setter TargetName="Border" Property="BorderBrush" Value="Transparent"/>
                </Trigger>
                <Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="True">
                    <Setter TargetName="SubmenuBorder" Property="CornerRadius" Value="0,0,4,4"/>
                    <Setter TargetName="SubmenuBorder" Property="Padding" Value="0,0,0,3"/>
                </Trigger>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter Property="Foreground" Value="#888888"/>
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>

        <ControlTemplate x:Key="{x:Static MenuItem.SubmenuHeaderTemplateKey}" TargetType="{x:Type MenuItem}">
            <Border Name="Border" >
                <Grid>
                    <ContentPresenter
        Margin="6,3,6,3"
        ContentSource="Header"
        RecognizesAccessKey="True" />
                    <Popup
      Name="Popup"
      Placement="Right"
      IsOpen="{TemplateBinding IsSubmenuOpen}"
      AllowsTransparency="True"
      Focusable="False"
      PopupAnimation="Fade">
                        <Border
        Name="SubmenuBorder"
        SnapsToDevicePixels="True"
        Background="#0D000000">
                            <StackPanel
          IsItemsHost="True"
          KeyboardNavigation.DirectionalNavigation="Cycle" />
                        </Border>
                    </Popup>
                </Grid>
            </Border>
            <ControlTemplate.Triggers>
                <Trigger Property="IsSuspendingPopupAnimation" Value="true">
                    <Setter TargetName="Popup" Property="PopupAnimation" Value="None"/>
                </Trigger>
                <Trigger Property="IsHighlighted" Value="true">
                    <Setter TargetName="Border" Property="Background" Value="#C0C0C0"/>
                    <Setter TargetName="Border" Property="BorderBrush" Value="Transparent"/>
                </Trigger>
                <Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="True">
                    <Setter TargetName="SubmenuBorder" Property="CornerRadius" Value="0,0,4,4"/>
                    <Setter TargetName="SubmenuBorder" Property="Padding" Value="0,0,0,3"/>
                </Trigger>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter Property="Foreground" Value="#888888"/>
                </Trigger>
            </ControlTemplate.Triggers>



        </ControlTemplate>