How to use a different theme for a contribution page using Drupal
Themekey does indeed support this, as long as you are using Drupal.  I couldn't get this to work until I found that as well as creating a record of the page and the theme you want to use, you have to do a bit more.  You must have 'Themekey CiviCRM Integration' installed and you have to first go to Configuration/Themekey/Settings/Compatability then check 'CiviCRM Theme (Define alternate themes for CiviCRM.)'  See also my comments.