Hi ,
This is the site that allows you to build a custom skin.
http://stylebuilder.telerik.com/
This is how we explain how to do that in the Users Guide doc.
Please let me know if you have any questions.
"Allows you to select the skin name for the menu. If you have a custom
skin, you can add it to the following folder for SharePoint
2010:C:\Program Files\Common Files\Microsoft Shared\Web Server
Extensions\14\TEMPLATE\LAYOUTS\QuestSoftware\Telerik\Q32012\Skins
or for SharePoint 2013: C:\Program Files\Common Files\Microsoft
Shared\Web Server
Extensions\15\TEMPLATE\LAYOUTS\QuestSoftware\Telerik\Q32012\Skins
Note: When creating a custom skin, you can refer to the ple Skin
folder in the install directory."