In order to use the built-in skins for RadControls you ave to make sure that the Telerik.Web.UI.Skins.dll is included/referred to your project. You ca find detailed information regarding the Skins and how to use them in the following section of our documentation: Controlling Visual Appearance
Additionally, there is an Appearance and styling section for each RadControl, where the skin specifics are described. For instance, the Slider's one could be found here: Appearance and Styling
Hope this helps.
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