I would have thought of something small that could perhaps be integrated with little effort.
You know my dark theme here in the community. How would it be to use this as a kind of day/night theme?
All that would be necessary would be two links in the user menu with one link pointing to the Codoforum standard theme (i.e. light design / day mode) and you could add my theme to the system as a theme and use it as a dark theme via a link. So all you have to do is integrate these two links into the user menu and the theme chosen by the user is then stored in the browser cookie. Bingo ready is the Codoforum Day/Night Mode
That would be a little thing that many would be happy about and for me it's ok if you use my theme for it.