Please also see the following stack over flow issue.
it is related to them
Now here is solution :
Remove the files of the bad theme and clear the cache. After clearing the cache you will be able to login again.
The main difficulty is that you have to clear the cache without being logged in.
Try one of the methods for clearing the cache described in
Clearing Drupal's cache
IF Not then Try this one :
If you have drush, the command to type would be
drush vset theme_default garland
Either on the commandline, or via an administration interface (eg PHPMyAdmin) enter the following query
UPDATE system SET status=1 WHERE name = 'garland';
Then either:
UPDATE variable SET value='s:7:"garland"' WHERE name = 'theme_default';
TRUNCATE cache;
TRUNCATE cache_bootstrap;
TRUNCATE cache_block;
Note that 's:7' refers to the length of the following string. Modify as needed. This is database surgery, tricky stuff.
OR
If you are using per-user themes, and you've just messed it up for yourself as admin, try
UPDATE users SET theme='garland' WHERE uid = '1';
Be careful, as getting either of those lines wrong can mess things up just as badly.
Cheers!
Mudassar Ali