Blog Theme module for Drupal
The Blog Theme module allows individual users to select different themes for their personal blog pages. Drupal allows easy switching of themes in that there is a theming layer which is very flexible in making the presentation of a drupal site customizable. This Blog Theme module uses that facility to allow a different theme to be used for blog postings by each user.
It's not clear how much choice the users have to freely customize their theme. In use the choice is made by the user editing their account settings. The user can select between the themes the administrator has defined in the Drupal installation. It seems then that the users cannot upload their own theme, but then that capability if it existed would be a serious security hole.