One user stuck on (broken) dark mode; only in whiteboard, not in other apps

[woiwoiwoiwoi]

The title says it: one user is stuck on dark mode, but only in whiteboard, not in any of the other apps.

I have tried many browsers, private windows, Linux OS theme settings, browser theme settings, Nextcloud user theme settings, and Nextcloud admin theme settings. Nothing works. This one single user is stuck on dark mode. The odd thing is also that there are some imperfections - this for instance is the help menu of this user:

... while it looks like this for other users when dark mode is enabled:

The background black is also a slightly different black. I thought maybe that user has some custom theming enabled. But whiteboard was fine a few days ago and no theming has been changed in the meantime.

[woiwoiwoiwoi]

Just noticed that when dark mode is enabled for that user, it works correctly. But when light mode is enabled, it leads to the above issues.