Three styles, all applied implicitly by Styles/Controls.xaml: MahApps.Styles.Menu for the bar, MahApps.Styles.ContextMenu for the popup, and MahApps.Styles.MenuItem for what goes in either. The quick start is the whole setup.

<Menu>
    <MenuItem Header="_File" />
    <MenuItem Header="_Edit" />
    <MenuItem Header="_View" />
    <MenuItem Header="_Help" />
</Menu>

Menu items

MahApps.Styles.MenuItem picks a template per Role — top-level header, top-level item, submenu header, submenu item — and each of those handles the icon, the check mark, the gesture text and the submenu arrow:

<ContextMenu>
    <MenuItem Header="New" InputGestureText="Ctrl+N">
        <MenuItem.Icon>
            <TextBlock FontFamily="{DynamicResource MahApps.Fonts.Family.SymbolTheme}" Text="&#xE8A5;" />
        </MenuItem.Icon>
    </MenuItem>
    <MenuItem Header="Open" InputGestureText="Ctrl+O" />
    <Separator />
    <MenuItem Header="Word wrap" IsCheckable="True" IsChecked="True" />
    <MenuItem Header="Show line numbers" IsCheckable="True" />
    <Separator />
    <MenuItem Header="Export as">
        <MenuItem Header="PDF" />
        <MenuItem Header="HTML" />
    </MenuItem>
    <MenuItem Header="Print" InputGestureText="Ctrl+P" IsEnabled="False" />
</ContextMenu>

Everything in that figure comes out of the box — none of it needs a style of its own. The columns line up because the ContextMenu style sets Grid.IsSharedSizeScope="True", so the icon, text and gesture columns are measured across all the items at once. InputGestureText is only a label: it does not create the shortcut, your KeyBinding or RoutedCommand does.

Padding differs by role — 7 5 8 6 for the two top-level ones, 2 6 2 6 in a submenu — which is why a bar item is wider than tall and a popup item the other way round.

The separator is styled through {x:Static MenuItem.SeparatorStyleKey} and is a two-pixel groove inset 20 units from the left, so it starts past the icon column rather than cutting across it.

The popup

MahApps.Styles.ContextMenu sets HasDropShadow="True" and its template applies MahApps.DropShadowEffect.Menu, offsetting the border by 0 0 6 6 to leave room for it. It also sets OverridesDefaultStyle="True", so it does not inherit anything from WPF's own context menu style.

System keys

Both styles reach for system resources rather than theme ones:

<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.MenuTextBrushKey}}" />
<Setter Property="FontFamily" Value="{DynamicResource {x:Static SystemFonts.MenuFontFamilyKey}}" />

The colour still follows the theme, because MahApps redefines that system key in the theme dictionary — SystemColors.MenuTextBrushKey is given MahApps.Colors.ThemeForeground there. Change the base theme and menu text changes with it.

The fonts are a different matter. MahApps does not redefine the SystemFonts.Menu* keys, so the family, style and weight of menu text come from the reader's Windows settings, not from Fonts.xaml. Menus are the one place in a MahApps window where that is true, and it is deliberate — menus are a shell affordance. Only the size is the library's, from MahApps.Font.Size.Menu and .ContextMenu.

The text box context menu

Controls.ContextMenu.xaml also holds a ready-made menu:

<ContextMenu x:Key="MahApps.TextBox.ContextMenu" x:Shared="False">
    <MenuItem Command="ApplicationCommands.Cut" />
    <MenuItem Command="ApplicationCommands.Copy" />
    <MenuItem Command="ApplicationCommands.Paste" />
</ContextMenu>

The styles for TextBox, PasswordBox, DatePicker, DateTimePicker and HotKeyBox all point their ContextMenu at it. x:Shared="False" gives each control its own instance, which a context menu needs.

Redefining that key in App.xaml — after the MahApps dictionaries — swaps the context menu of every one of those controls at once, which is easier than setting ContextMenu on each.

Spell checking

TextBoxHelper.IsSpellCheckContextMenuEnabled turns on the spell-check entries for a TextBox or RichTextBox, and MahApps styles them through four ComponentResourceKeys on mah:Spelling: SuggestionMenuItemStyleKey (bold, header bound to the suggestion), IgnoreAllMenuItemStyleKey, NoSuggestionsMenuItemStyleKey and SeparatorStyleKey.

<TextBox SpellCheck.IsEnabled="True"
         mah:TextBoxHelper.IsSpellCheckContextMenuEnabled="True" />

See TextBoxHelper.

Visual Studio variants

Styles/VS/ holds MahApps.Styles.Menu.VisualStudio, MahApps.Styles.MenuItem.VisualStudio and MahApps.Styles.ContextMenu.VisualStudio, plus its own MenuItem.SeparatorStyleKey and its own MahApps.TextBox.ContextMenu.

Items from a CompositeCollection

To share part of a menu, declare it as a CompositeCollection resource and pull it in through ItemsSource:

<CompositeCollection x:Key="ContextMenuBase" x:Shared="False">
    <MenuItem Command="ApplicationCommands.New" />
    <MenuItem Command="ApplicationCommands.Delete" />
    <Separator />
</CompositeCollection>

<ContextMenu>
    <ContextMenu.ItemsSource>
        <CompositeCollection>
            <CollectionContainer Collection="{StaticResource ContextMenuBase}" />
            <MenuItem Command="ApplicationCommands.Print" />
        </CompositeCollection>
    </ContextMenu.ItemsSource>
</ContextMenu>

That works, but floods the output window:

The two bindings it is complaining about are in MahApps.Styles.MenuItem itself:

<Setter Property="HorizontalContentAlignment" Value="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}" />
<Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}" />

They are what lets a ContextMenu set the alignment for all of its items — the same pattern WPF's own item styles use. A MenuItem declared inside a CompositeCollection resource, though, is built before it belongs to anything, so the ancestor lookup finds no ItemsControl and logs. The menu itself is fine; only the log is noisy.

Replacing the two bindings with fixed values silences it:

<Style BasedOn="{StaticResource {x:Type MenuItem}}" TargetType="{x:Type MenuItem}">
    <Setter Property="HorizontalContentAlignment" Value="Left" />
    <Setter Property="VerticalContentAlignment" Value="Center" />
</Style>

The cost is the feature those bindings provided: alignment set on a ContextMenu no longer reaches its items.