Not available at this time, and may not be incorporated into the completed version in the same way (if at all).
You should be able to go to "C:\Program Files\Stardock\Object Desktop\WindowBlinds" folder, and delete the skin folder - then start WB 5 and click the refresh icon below the preview window. Note - I have found that in some cases the skin list disappears until I move the scroll bar below the skin list, which then redraws the skin list images.
The option to delete an already installed skin is located in the Preview image, just below "Change Skin Colour" and above "Apply this skin now".
Do not delete skins manually from explorer. The delete option in wbconfig should be used.