With firebug you don`t have to edit the embedded css styles which are default for the navigation control. You have to use the classes like .rmSelected and add different rules to them with !important where needed. You style the menu in your theme`s css or you put the css in a CSS widget. Please note firebug only apply test styles and changes to the styles are not saved in your css files. To edit your css and save it to the file as you browse you need to use other plugins.
All the best,
the Telerik team
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