I just spent the whole day trouble shooting one problem in a custom theme. Galleria would throw a "Theme CSS could not load" 95% of the time when my site was viewed on a mobile device. The weird part is that the other 5% of the time was almost completely random.
Long story short, go into your custom theme's js and min.js and remove the CSS reference (ex. css: 'galleria.classic2.css',). Then go into your Drupal theme and add that CSS file to your themes .info file. BAM! everything solved.
I'm going to file this as a bug report, but it might also be a feature request and a documentation issue. Please correct accordingly.