inabox/inabox.wpf/DynamicGrid/Themes/Main.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:Class="InABox.DynamicGrid.MainResources">

    <Style x:Key="NonHighlightMenuItem" TargetType="{x:Type MenuItem}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type MenuItem}">
                    <Border Background="{TemplateBinding Background}">
                        <ContentPresenter Content="{TemplateBinding Header}" Margin="35 5 10 5"/>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsHighlighted" Value="True">
                            <Setter Property="Background" Value="Transparent"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

</ResourceDictionary>