You might have gotten a bad download. Apply a different skin, then delete Darkness. Download it again and see if that fixes it.

i had this problem with windowsblinds pnce and an update seemed to fix it, but if you still get the problem then this workaround that i used may work for you. rename the themes extension to .zip (.wba files are just renamed .zips after all) then extract the files contained within. Then place that folder into c:/documents/stardock/windowblinds ( this is on windows 7 alter for whereever vista stores the windowblinds files). then load the windowblinds confiuration window and click skin options...rebuild list. Once that is done the theme should then appear in the list of themes available. alternatively you can run impulse (or download and install it if youi havent already) and check for any windowblinds updates and update to the latest version if you are able to. Good luck