The ability to use a mobile friendly theme for a contribution page is vital these days. I used 'Themekey CiviCRM' along with Themekey which worked for a while but doesn't anymore. Fixing that would be good but it seems a bit of a cludge. Isn't there a better solution for this common requirement? Or should I be using a webform rather than the contribution page? I'm using 'Seven' as the normal theme for the back office functions, and don't want to use a different theme for ALL public pages as then CiviVolunteer pages are themed differently. On 4.7.24 and 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.