You can use a custom or a modified skin but you will have to register the skin CSS file by hand. The skin CSS file can be registered declaratively or from code-behind. In your case I suggest using the Vista skin and modify it so that it serves your needs. For reference on how skins in RadControls "Prometheus" work and how to modify an existing skin or create a custom skin and then register it I suggest reviewing the following the following help articles:
In case you experience problems, do not hesitate to contact us again.
the Telerik team