I am using Firefox 97.0.1 on Windows and a similar version of Firefox on Linux. Whichever of those setups that I use, the theme for the forum seems to be stuck on ‘light’. On the forum website I did the settings for my user. Therein ‘theme’ is set to ‘dark’. I toggled that, i.e. set it to light, applied the setting, and then reset it to dark and applied that setting. No dice. Perhaps I make some mistake.