Interesting bug: Single user can't change themes

Here’s a fun one. On my forum I have one single user with a really weird problem-- when he changes themes, the color scheme doesn’t change with the theme! The same themes work for thousands of others, only his user has this problem, and only on desktop, mobile works fine.

I tried impersonating the user and changing all the settings in interface customization, no dice. The only way I could avoid the problem was enabling user-selectable color schemes (as opposed to themes), then impersonating and setting his user to use the correct colors for the theme he wants. This means he can’t switch themes on his own, ever, so it isn’t a great solution.

Is this a known bug? Any way to fix it?

What happens if he tries another browser?

Same deal, and the same when I impersonate him also.