I suggest you examine the 'Controlling Visual Appearance' chapter from the online documentation as a starting point in familiarizing with the skinning mechanism exposed for our controls:
As well as the individual skinning topics from each control's documentation. They can give you the foundation on which you can design your appearance customization model for our ASP.NET AJAX controls.
Furthermore, I encourage you to try out our Visual Style Builder tool which has been announced recently:
It can be used to create variations of the existing skins as well as building new skins from scratch without the necessity to become a css expert or have profound css knowledge.
the Telerik team