If this problem is present for all skins as indicated by sdRohan, I would recommend:
1. Open WindowBlinds and choose the Default theme at top of the list and apply it.
3. De-install WindowBlinds ( I am assuming you have an active account and can re-download WindowBlinds)
5. Go to your Stardock Store Account and re-download the latest version of WindowBlinds.
6. Install WindowBlinds.
All should be well if the problem was a corrupted install of WindowBlinds. Let us know if this solves the problem.