Let users download and set induvidual blog themes Drupal 7
Letting them download and put up whatever theme that they want sounds like it might be a major security issue for your site, since they could put whatever PHP that they want into the theme files and it would run. I'd suggest making a specific set of themes you have picked available and letting users choose from those. There are modules like Theme key to enable using different themes: http://drupal.org/project/themekey