In case you do not want to use one of the embedded skins of our controls and would like to customize them through your own css classes only, consider setting the Skin property of the respective controls to an empty string and use the exposed css classes for their elements to style them. Here are a couple of help articles (for RadInput and RadMenu) concerning this subject:
On a side note, have you considered using our Visual Style Builder for ASP.NET announced recently:
It allows you to fine-tune/customize the existing skins of our controls or create a new custom skin in several easy steps without the need of extensive CSS/designer skills. I encourage you to try it out and share your feedback with us.
the Telerik team