The updated version, 1.1.0, of the WonderCMS Bootstrap 5 theme includes support for “Dark mode”, based on the visitor browser’s preference.
If you haven’t yet checked out WonderCMS, and if you’re looking for a really tiny CMS platform, you may want to have a look.
The files for the theme is in my GitHub repo github.com/joho1968/wcms-bs5. You can see it in action here wondercms.oppetmoln.se
(Where you can also download a .zip file.)
Enjoy.