It's most likely the SharedSettings.ccs that you find in c:\users\[your user name]\AppData\roaming. But wait! don't delete it, you may lose other important stuff, like all the server settings in DFTP. Instead, just move it to somewhere else (desktop), and then let it be recreated by the system. Reinstall the programme, and then hopefully you have your themes back. Then it should be possible to move the original SharedSettings.ccs back.
I have used this method a couple of times with RSD, where I also have had themes or components missing, and it has always worked just fine.
Have tried this three times - including restarts in-between - no luck. It is just the same - blank theme window and a corrupt file message if i try to import direct from the 'themes directory'.
Should i put the Shared Settings.css back?