Ok, 2 things
1) I have a plugin that allows you to assign categories and tags to pages. I had hoped this would trigger Background Manager to set a per-category background to those pages too, but that doesn't seem to work. If anyone knows a way to do that, please let me know
Then, I switched to choosing an image-set per page, but that brings this problem:
2) I use transparent backgrounds and specify the override color per set. Now, when I choose a set for a specific page, it displays the backgrounds in that set but not the override color I set for it. I have to set the override color manually on every page. This looks like a bug.