Enable dark mode
Bootstrap has it, with <html data-bs-theme="dark">.
It does not support automatically setting it from browser preferences, without editing SASS.
We can store the theme preference in a cookie, and add a user menu entry to toggle between the two.
If JavaScript exists, and there is no preferred theme stored in the cookie, then read the preference from the browser and use that, providing it can be done early enough not to cause a page flicker