You can install drush on your system and then from drush command you can change the theme using

drush vset theme_default <theme_name>

You can install drush on your system and then from drush command you can change the theme using

drush vset theme_default <theme_name>

You can do the changes from database also. Either on the commandline, or via an administration interface (eg PHPMyAdmin) enter the following query

UPDATE system SET status=1 WHERE name = '<theme_name>';

Then either:

UPDATE variable SET value='s:7:"garland"' WHERE name = 'theme_default';
TRUNCATE cache;
TRUNCATE cache_bootstrap;
TRUNCATE cache_block;

Source: https://drupal.org/node/200774