I guess this should really be an FAQ: Keep a theme around that's known to work, e.g. the Professional theme. If you run into problems as above, set the default theme in config.php back to that working theme and rename the directory of the non-working theme (to whatever you want - just so that it has a new name). Geeklog will then pick up the default theme again, even if you've selected the faulty theme in your preferences.
bye, Dirk