In order to remove the border, you have to edit the control template, which contains a Border.
<Border x:Name="SubMenuBorder" Background="{StaticResource Menu.Static.Background}" BorderThickness="0" BorderBrush="{StaticResource Menu.Static.Border}" Padding="2">
The border that you notice is actually caused by the hard-coded BorderThicknessand the Padding.
<Border x:Name="SubMenuBorder" Background="{StaticResource Menu.Static.Background}">
If you remove those for the top level header template and submenu header template in the default style for MenuItem (see code below), the visible border is gone as you can see below.
You can extract the default styles and templates for MenuItem using Blend or Visual Studio. Then you only have to adapt the lines above. This is the adapted XAML code for the styles and templates:
<SolidColorBrush x:Key="Menu.Static.Background" Color="#FFF0F0F0"/>
<SolidColorBrush x:Key="Menu.Static.Border" Color="#FF999999"/>
<SolidColorBrush x:Key="Menu.Static.Foreground" Color="#FF212121"/>
<SolidColorBrush x:Key="Menu.Static.Separator" Color="#FFD7D7D7"/>
<SolidColorBrush x:Key="Menu.Disabled.Foreground" Color="#FF707070"/>
<SolidColorBrush x:Key="MenuItem.Selected.Background" Color="#3D26A0DA"/>
<SolidColorBrush x:Key="MenuItem.Selected.Border" Color="#FF26A0DA"/>
<SolidColorBrush x:Key="MenuItem.Highlight.Background" Color="#3D26A0DA"/>
<SolidColorBrush x:Key="MenuItem.Highlight.Border" Color="#FF26A0DA"/>
<SolidColorBrush x:Key="MenuItem.Highlight.Disabled.Background" Color="#0A000000"/>
<SolidColorBrush x:Key="MenuItem.Highlight.Disabled.Border" Color="#21000000"/>
<MenuScrollingVisibilityConverter x:Key="MenuScrollingVisibilityConverter"/>
<Geometry x:Key="DownArrow">M 0,0 L 3.5,4 L 7,0 Z</Geometry>
<Geometry x:Key="UpArrow">M 0,4 L 3.5,0 L 7,4 Z</Geometry>
<Geometry x:Key="RightArrow">M 0,0 L 4,3.5 L 0,7 Z</Geometry>
<Geometry x:Key="Checkmark">F1 M 10.0,1.2 L 4.7,9.1 L 4.5,9.1 L 0,5.2 L 1.3,3.5 L 4.3,6.1L 8.3,0 L 10.0,1.2 Z</Geometry>
<Style x:Key="MenuScrollButton" BasedOn="{x:Null}" TargetType="{x:Type RepeatButton}">
<Setter Property="ClickMode" Value="Hover"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border x:Name="templateRoot" Background="Transparent" BorderThickness="1" BorderBrush="Transparent" SnapsToDevicePixels="true">
<ContentPresenter HorizontalAlignment="Center" Margin="6" VerticalAlignment="Center"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="{ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}" BasedOn="{x:Null}" TargetType="{x:Type ScrollViewer}">
<Setter Property="HorizontalScrollBarVisibility" Value="Hidden"/>
<Setter Property="VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollViewer}">
<Grid SnapsToDevicePixels="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Border Grid.Column="0" Grid.Row="1">
<ScrollContentPresenter CanContentScroll="{TemplateBinding CanContentScroll}" Margin="{TemplateBinding Padding}"/>
</Border>
<RepeatButton Command="{x:Static ScrollBar.LineUpCommand}" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Grid.Column="0" Focusable="false" Grid.Row="0" Style="{StaticResource MenuScrollButton}">
<RepeatButton.Visibility>
<MultiBinding Converter="{StaticResource MenuScrollingVisibilityConverter}" ConverterParameter="0" FallbackValue="Visibility.Collapsed">
<Binding Path="ComputedVerticalScrollBarVisibility" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="VerticalOffset" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ExtentHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ViewportHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
</MultiBinding>
</RepeatButton.Visibility>
<Path Data="{StaticResource UpArrow}" Fill="{StaticResource Menu.Static.Foreground}"/>
</RepeatButton>
<RepeatButton Command="{x:Static ScrollBar.LineDownCommand}" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Grid.Column="0" Focusable="false" Grid.Row="2" Style="{StaticResource MenuScrollButton}">
<RepeatButton.Visibility>
<MultiBinding Converter="{StaticResource MenuScrollingVisibilityConverter}" ConverterParameter="100" FallbackValue="Visibility.Collapsed">
<Binding Path="ComputedVerticalScrollBarVisibility" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="VerticalOffset" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ExtentHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ViewportHeight" RelativeSource="{RelativeSource TemplatedParent}"/>
</MultiBinding>
</RepeatButton.Visibility>
<Path Data="{StaticResource DownArrow}" Fill="{StaticResource Menu.Static.Foreground}"/>
</RepeatButton>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=TopLevelItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" SnapsToDevicePixels="true">
<Grid VerticalAlignment="Center">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Path x:Name="GlyphPanel" Data="{StaticResource Checkmark}" FlowDirection="LeftToRight" Fill="{StaticResource Menu.Static.Foreground}" Margin="3" VerticalAlignment="Center" Visibility="Collapsed"/>
<ContentPresenter ContentSource="Header" Grid.Column="1" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="GlyphPanel" Value="{StaticResource Menu.Disabled.Foreground}"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsHighlighted" Value="True"/>
<Condition Property="IsEnabled" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Disabled.Border}"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=TopLevelHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" SnapsToDevicePixels="true">
<Grid VerticalAlignment="Center">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Path x:Name="GlyphPanel" Data="{StaticResource Checkmark}" FlowDirection="LeftToRight" Fill="{TemplateBinding Foreground}" Margin="3" VerticalAlignment="Center" Visibility="Collapsed"/>
<ContentPresenter ContentSource="Header" Grid.Column="1" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
<Popup x:Name="PART_Popup" AllowsTransparency="true" Focusable="false" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" Placement="Bottom" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" PlacementTarget="{Binding ElementName=templateRoot}">
<Border x:Name="SubMenuBorder" Background="{StaticResource Menu.Static.Background}">
<ScrollViewer x:Name="SubMenuScrollViewer" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=SubMenuBorder}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<Rectangle Fill="{StaticResource Menu.Static.Separator}" HorizontalAlignment="Left" Margin="29,2,0,2" Width="1"/>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="true" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="true">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="GlyphPanel" Value="{StaticResource Menu.Disabled.Foreground}"/>
</Trigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="SubMenuScrollViewer" Value="false">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=SubmenuItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" Height="22" SnapsToDevicePixels="true">
<Grid Margin="-1">
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="22" SharedSizeGroup="MenuItemIconColumnGroup" Width="Auto"/>
<ColumnDefinition Width="13"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="30"/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
<ColumnDefinition Width="20"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Border x:Name="GlyphPanel" Background="{StaticResource MenuItem.Selected.Background}" BorderThickness="1" BorderBrush="{StaticResource MenuItem.Selected.Border}" ClipToBounds="False" HorizontalAlignment="Center" Height="22" Margin="-1,0,0,0" VerticalAlignment="Center" Visibility="Hidden" Width="22">
<Path x:Name="Glyph" Data="{StaticResource Checkmark}" FlowDirection="LeftToRight" Fill="{StaticResource Menu.Static.Foreground}" Height="11" Width="10"/>
</Border>
<ContentPresenter x:Name="menuHeaderContainer" ContentSource="Header" Grid.Column="2" HorizontalAlignment="Left" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center"/>
<TextBlock x:Name="menuGestureText" Grid.Column="4" Margin="{TemplateBinding Padding}" Opacity="0.7" Text="{TemplateBinding InputGestureText}" VerticalAlignment="Center"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="Glyph" Value="{StaticResource Menu.Disabled.Foreground}"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsHighlighted" Value="True"/>
<Condition Property="IsEnabled" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Disabled.Border}"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<ControlTemplate x:Key="{ComponentResourceKey ResourceId=SubmenuHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" Height="22" SnapsToDevicePixels="true">
<Grid Margin="-1">
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="22" SharedSizeGroup="MenuItemIconColumnGroup" Width="Auto"/>
<ColumnDefinition Width="13"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="30"/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
<ColumnDefinition Width="20"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Border x:Name="GlyphPanel" Background="{StaticResource MenuItem.Highlight.Background}" BorderThickness="1" BorderBrush="{StaticResource MenuItem.Highlight.Border}" Height="22" Margin="-1,0,0,0" VerticalAlignment="Center" Visibility="Hidden" Width="22">
<Path x:Name="Glyph" Data="{DynamicResource Checkmark}" FlowDirection="LeftToRight" Fill="{StaticResource Menu.Static.Foreground}" Height="11" Width="9"/>
</Border>
<ContentPresenter ContentSource="Header" Grid.Column="2" HorizontalAlignment="Left" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center"/>
<TextBlock Grid.Column="4" Margin="{TemplateBinding Padding}" Opacity="0.7" Text="{TemplateBinding InputGestureText}" VerticalAlignment="Center"/>
<Path x:Name="RightArrow" Grid.Column="5" Data="{StaticResource RightArrow}" Fill="{StaticResource Menu.Static.Foreground}" HorizontalAlignment="Left" Margin="10,0,0,0" VerticalAlignment="Center"/>
<Popup x:Name="PART_Popup" AllowsTransparency="true" Focusable="false" HorizontalOffset="-2" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" Placement="Right" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" VerticalOffset="-3">
<Border x:Name="SubMenuBorder" Background="{StaticResource Menu.Static.Background}">
<ScrollViewer x:Name="SubMenuScrollViewer" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=SubMenuBorder}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<Rectangle Fill="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}" HorizontalAlignment="Left" Margin="29,2,0,2" Width="1"/>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="true" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="true">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="Transparent"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="{StaticResource MenuItem.Highlight.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="Glyph" Value="{StaticResource Menu.Disabled.Foreground}"/>
<Setter Property="Fill" TargetName="RightArrow" Value="{StaticResource Menu.Disabled.Foreground}"/>
</Trigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="SubMenuScrollViewer" Value="false">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Style TargetType="{x:Type MenuItem}">
<Setter Property="HorizontalContentAlignment" Value="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderBrush" Value="Transparent"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="ScrollViewer.PanningMode" Value="Both"/>
<Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
<Setter Property="Template" Value="{DynamicResource {ComponentResourceKey ResourceId=SubmenuItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}}"/>
<Style.Triggers>
<Trigger Property="Role" Value="TopLevelHeader">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderBrush" Value="Transparent"/>
<Setter Property="Foreground" Value="{StaticResource Menu.Static.Foreground}"/>
<Setter Property="Template" Value="{DynamicResource {ComponentResourceKey ResourceId=TopLevelHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}}"/>
<Setter Property="Padding" Value="6,0"/>
</Trigger>MenuItem
<Trigger Property="Role" Value="TopLevelItem">
<Setter Property="Background" Value="{StaticResource Menu.Static.Background}"/>
<Setter Property="BorderBrush" Value="{StaticResource Menu.Static.Border}"/>
<Setter Property="Foreground" Value="{StaticResource Menu.Static.Foreground}"/>
<Setter Property="Template" Value="{DynamicResource {ComponentResourceKey ResourceId=TopLevelItemTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}}"/>
<Setter Property="Padding" Value="6,0"/>
</Trigger>
<Trigger Property="Role" Value="SubmenuHeader">
<Setter Property="Template" Value="{DynamicResource {ComponentResourceKey ResourceId=SubmenuHeaderTemplateKey, TypeInTargetAssembly={x:Type MenuItem}}}"/>
</Trigger>
</Style.Triggers>
</Style>
Copy the code to a resource dictionary in scope. The style for MenuItem is implicit, so it is automatically applied to all MenuItems in scope. If you want to apply this style only selectively, add an x:Key to make it explicit and reference it via the StaticResource or DynamicResource markup extensions.
Of course, you could roll your own custom menu item style using guidance from the documentation on styles and templates for MenuItem that features required states and parts, but that is a complex task and prone to errors, so it is way easier to copy and adapt the default styles.
