For some reason my themes won't switch. Permissions to all themes are set at 0775 but everytime I click "Use this theme" nothing happens. It seems stuck on the default theme. Any ideas? Cheers

Did you remember to set the permissions correct for your cache directory?
Information about the active theme is stored in a .txt file within the cache directory. It has nothing to do with the permissions on the themes themselves.

try to erase all the content of the temp dir and after try to change the theme

Yep, a text file is kind of a bad way to store it, but that's how it's currently done.

We need an options database table...

Oh, which means try deleting theme.txt, give world write perms to it, or to the /cache directory.