I have the default theme, afaik. I could maybe try to install one or deinstall it or something. Maybe that would reset the tab settings. I'm not aware of any accessibility changes. There was a Firefox automated update not that long ago, but I don't think that caused the color switch and I just didn't notice for a day and half. Safe-mode doesn't fix the problem. I'd try reset, but then I'd loose all my beautifully sorted tab groups that I use to keep track of fanfic to read.