I'm doing some guessing here, since I don't have all the dots connected about how the whole webtrees app is "wired"...

Anyway, from Bootstrap 5.3 light or dark color modes can be selected. I think it would be useful to be able to set this in the theme interface, to tell if the theme wants light or dark "base mode". This interface extension would then conditionally add data-bs-theme="light" or data-bs-theme="dark" to the html element.
This change would make it easier to implement dark themes for webtrees.