The visual appearance of Telerik RadControls "Prometheus" is defined through "skins". A skin consists of a CSS file and image files (which are optional). Telerik RadControls "Prometheus" come with a set of built-in skins which are ready to be used. The user can use the built-in skins as they are or customize them.
For details on the matter please refer to the controls' documentation under section RadControls 'Prometheus' Fundamentals --> Controlling Visual Appearance
the Telerik team