I haven't looked too deeply into this but choosing a new color scheme causes no changes upon hitting the Set Theme button for me in Safari.

I haven't gotten to the bottom of it but I think it may be due to changes in July, but it also might not be, since I think this just doesn't get a ton of attention and these could just be unrelated oversights/noise: The console is throwing obvious errors due to color_scheme_fish_default being missing (it used to be in colorutils.js, now it's nowhere), and a random static analysis tool¹ has 4 alerts to complain about between webconfig.py and controller.js on one commit from July… which might also not actually be the cause of the problem.

I like to imagine if this didn't work for four months people would notice. So it really could be something else.