maintenance-page--offline template not working
This is caused when class_exists() is being called from within alpha_get_theme() and autoloading fails. I have a patch that fixes this by wrapping those calls in a try/catch block. I'm not quite sure what should be done in the case of an error though, so I have just done nothing.

See theme Omega issue => https://www.drupal.org/node/1722586

It is working for me, I had the same problem.