I just tried to edit the backgrounds in a couple of older comics (373745 and 366696), and found that I can't change the category (it doesn't stay after you press 'update'). I can change the backgrounds within the same category though.

I also noticed that when I try the background changes to pure white, but when I go back in to re-edit it shows the first background of the original category on the drop-down, while not displaying anything in the preview (if you do any testing, don't try it with something that had 'none' as the background before, as it will appear just to return to the original).

I can then change it back to the original background, or any of the ones in that category without any problems.

I think this should be fixed. Want to give it a try, let me know how it goes?

That sounds like the same problem I was having, and it's fixed for me now.

Yep, all working. Thanks Brad!