I have been experiencing the same issue , and I can assure you that it's not due to cache overload or anything else that can be fixed from the client side.
The themes in Power Toggles are actually just background images specifically designed to fit nicely in their widget, and i'm pretty sure that "Painless" (the app's developer) has just stopped hosting the image files on their server. If you get in touch with them (ie leave a nasty review or something), they might be able to help. Its just a matter of finding those jpg or PNG image files, and using them via the "Custom Background Image" option.
Hope this helps.