Sometimes a drupal theme can be broken and it can be needed to reset the default theme so the site can be accessed again in order to fix the problem.
As a rule of thumb I always try to use the theme "Cube" for the administrive part of drupal. that makes things a lot easier and if you breeak the theme you can at least still use the admin part.
Here is anyhow how to reset the theme using SQL:
Simply execute the three following commands:
UPDATE system SET status=1 WHERE name = 'garland';
UPDATE variable SET value='s:7:"garland"' WHERE name = 'theme_default';