In the appearances settings, the blue option does not work. It makes the icon white. All other colors work fine. I realize this isn't an earthshattering issue, but still.
What did you already try to solve it?
This happens in both separate installs I have, one on Pi 3b and one on a Pi 4.
Have you tried different browsers? These appearance settings haven't been touched in a long time (literally years) so it's unlikely there's a 'new' bug caused by OctoPrint here, it's either always been broken or there's a new browser behaviour.