To style any of the navigation types you should use the "Wrapper CSS class" in the settings of the navigation widget. There you should enter the suffix of your selector. For example if you enter "Stefano" and you are using the "horizontal" type your selector will be "RadTabStrip_Stefano". Also this will disable the built-in styles.
Also don't put fixed height on containers - this will cause some problems in the page editor. You can use line-height instead.
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