Download for example "garland" theme to ./sites/all/themes folder from
https://www.drupal.org/project/garland
Then:
UPDATE dr_system SET status=1 WHERE name = 'garland';
Then:
UPDATE dr_variable SET value='s:7:"garland"' WHERE name = 'theme_default'; TRUNCATE dr_cache; TRUNCATE dr_cache_bootstrap; TRUNCATE dr_cache_block;
Here is Drush method
drush vset theme_default garland