<Style x:Key="{x:Type MenuItem}" TargetType="{x:Type MenuItem}">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Style.Triggers>
<Trigger Property="Role" Value="TopLevelHeader">
<Setter Property="Template" Value="{StaticResource {x:Static MenuItem.TopLevelHeaderTemplateKey}}"/>
<Setter Property="Grid.IsSharedSizeScope" Value="true"/>
</Trigger>
</Style.Triggers>
<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="{DynamicResource NormalBrush}"
BorderBrush="{StaticResource SolidBorderBrush}"
BorderThickness="1" >
<ScrollViewer CanContentScroll="True"
Style="{StaticResource MenuScrollViewer}">
<StackPanel
IsItemsHost="True"
KeyboardNavigation.DirectionalNavigation="Cycle" />
</ScrollViewer>
</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="{StaticResource NormalBrush}"/>
<Setter TargetName="Border" Property="BorderBrush" Value="Transparent"/>
</Trigger>
<!--Snippettoplevelheader-->
<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>
<!--/Snippettoplevelheader-->
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Foreground" Value="{StaticResource DisabledForegroundBrush}"/>
</Trigger>
</ControlTemplate.Triggers>
Триггер IsHighlighted определяет стиль при наведении курсора мыши. Если удалить триггер, при наведении курсора мыши на верхнее меню ничего не отображается
См. http://msdn.microsoft.com/en-us/library/ms771597%28VS.85%29.aspx для большего количества примеров стилей управления