Changing the color scheme per user is straightforward on a desktop browser, and slightly different on mobile browsers (or when installed as an app), but changing back to the default theme on mobile is only possible when desktop site is requested (even this only worked in Firefox, not in Chrome).
Is this a bug or am I doing something wrong?
What version of Discourse are you running? I think this bug is already fixed. I can reproduce it in a site running an older version, but it is solved in a site with the latest updates.
Can confirm that this is fixed as of 2.8.0.beta6 (I was on 2.8.0.beta5, so I just upgraded and tried again). Appreciate the quick fix, and wishing all of you a great weekend!