Have you both tried to change the theme from your Edit Your Options screen (under Miscellaneous options):

https://forums.psychcentral.com/prof...do=editoptions

Make sure you hit the "Save Changes" button at the bottom of that screen.

Going to a random post/thread that you've never viewed before, and trying to change the stylesheet from there seems to work more often for me, too. We haven't figured out what plugin is causing this odd behavior, unfortunately.