If you are working with RadMenu wrapped into a user control you can take a look at the following KB article: Controling appearance of RadControls for ASP.NET AJAX in Sitefinity
. In your case since you are working with multiple menus on the same page the best way to do this would be to:
1) Add your style sheet with custom styles, creating your custom skin as described in the artile, to the theme which your pages use.
2) Do not create .skin file in your theme folder, rather use each menu's edit tool to specify if you wish to use embedded skins, or if you wish to use custom skin as in attached image.
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