WPF C# - Change the brush of a menu's background

Does anyone know how to change the brush for a menu's background? This sounds simple, but I don't see any obvious way to do this. You'd think that the Background property would change it, but it doesn't.

Here's what my menu looks like (notice the default white background):

Example Code:

<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300">

<Window.Resources>
    <SolidColorBrush x:Key="menuItemBrush"
                     Color="#FF505050" />
</Window.Resources>

<Grid x:Name="mainGrid"
      Background="#FF252525">

    <Menu x:Name="mainMenu"
          Background="{DynamicResource menuItemBrush}"
          Grid.ColumnSpan="2"
          VerticalAlignment="Top">

        <MenuItem x:Name="fileMenu"
                  Background="{DynamicResource menuItemBrush}"
                  Foreground="White"
                  Header="File">

            <MenuItem x:Name="fileOpenMenuItem"
                      Background="{DynamicResource menuItemBrush}"
                      Foreground="White"
                      Header="Open..." />

            <MenuItem x:Name="fileSaveMenuItem"
                      Background="{DynamicResource menuItemBrush}"
                      Foreground="White"
                      Header="Save" />

            <MenuItem x:Name="fileSaveAsMenuItem"
                      Background="{DynamicResource menuItemBrush}"
                      Foreground="White"
                      Header="Save As..." />

            <Separator Style="{DynamicResource menuItemSeperator}" />

            <MenuItem x:Name="fileExitMenuItem"
                      Background="{DynamicResource menuItemBrush}"
                      Foreground="White"
                      Header="Exit" />
        </MenuI