Every so often I want to test a new theme on a live Drupal installation, but don’t want to break the website for anyone else. You can use ThemeKey to do this, although it is probably the simplest thing you can do with this module.
Navigate to http://example.com/admin/config/user-interface/themekey.
Create a new rule:
your user name
theme you want to see
You’ll be the only one who sees it. If you want to try out another theme, edit the rule.