This should not simply be based on the preferences of the operating system but have a toggle that allows you to change your opinion according to each site, as all sites have at the moment. It should not be automatic and irreversible but controllable. I.e.: using SessionStorage or Cookies or whatever you see it fits on purpose.
There is not the need to post this in the Ideas forum, it is not an idea, it is reality.
This request is for phpBB 4.0.0-a1 which has no dark mode switch and no dark mode at all. So it is not related to a hypothetical chameleon style/framework as it is still in its embryonic state and we don't know if and when it will become a butterfly.