Currently, we do not provide any tool to create or modify skins but we consider adding this in the future.
As of now we provide documentation for creating modifying skins available at www.telerik.com/skins
as well as in each particular product help (Controlling Visual Appearance section), for example RadGrid: http://www.telerik.com/help/aspnet/grid/?grdSkins.html
Please, do not hesitate to contact us if you have any specific questions for your custom skin.
We can not commit to a time-frame for providing such a tool but it is definitely in our ToDo list.
All the best,
the Telerik team