Go to http://demos.telerik.com/aspnet-ajax/menu/examples/functionality/rendermodes/defaultcs.aspx?Skin=Glow

Switch to RenderMode="Classic" and select some menu item. It will become gray colorized instead of yellow. The gradient should be removed.

Workaround:
Put the following style on the page with the menu control:
    <style>
        .rmLink.rmSelected { background-color: #db9810 !important; }
    </style>