Some users want to have dark mode, some users don't want it. Not all forum members have a same demand on forum displaying mode (dark or light).
Does it make any discomfort for anyone if there is a light/dark theme switcher? I think that absolutely everyone will be grateful, that's how I have made it in my BT template.
In summary: It's harder than it looks, it's probably not a good idea to begin with, and the work necessary to make it easy and sensible hasn't been done yet.
P.S. Don't be discouraged by my answer, the work you did is really cool, it's just that, when looked at from many angles, it's a very small part of the puzzle.
No, I am not discouraged in any way. Instead, I want to thank you again for everything that you have done for this forum and yes, I know what I did is a very small part of the puzzle. When I did it, my intention was to experience the responsive Bitcointalk on my devices, to see how this forum would look like in my smartphone if it was responsive. The only way to test that in real life was to write the template from scratch. It just then came as an idea for me to offer the community to make this change on the forum.