path_to_theme() call breaks site off-line page

Similar to #178364: path_to_theme() call breaking blocks page, the call to path_to_theme() in jlightbox_add() causes Drupal's site-off-line page to go completely un-themed. I believe this occurs because jlightbox_init() calls jlightbox_add(), and calls to theme functions in hook_init() are not supported.

See #219910: Calling theme function from hook_init() interferes with administration theme for more information.

To replicate:

1. Enable the jQuery Lightbox module.
2. Take your site off-line.
3. Log out.
4. You will be greeted by your site-off-line message in plain text, the page devoid of any theme/styling whatsoever.

The simple fix is to add an exception for when the site is off-line.