См. Этот вопрос: Прозрачное меню WPF
Вам необходимо перезаписать шаблон меню.Скопируйте / Вставьте это в ваш Window.Resources
или Menu.Resources
<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="Transparent">
<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>