(Long reading involved - see link)
As far as I know, there is no sign that Bootstrap 5 is officially supporting dark mode anytime soon.
My first suggestion is to add more ‘dark’ components to the existing 3 in BSS, but that will still limit the website to only one theme. If this is not the only feasible option, please read below.
If it’s possible for BSS devs to create just one component that converts everything added into it a hybrid (dark/light preferred), that would be excellent. This should not require a toggle switch (see Bootstrap 4 readme link at the end). Or a New Page option, maybe?
If both options are not feasible, perhaps a new premium(?) template inspired by the existing Grayscale could be considered. When creating a new page, instead of a blank black Body, maybe a cheatsheet like this one - Bootstrap 5 Dark - Cheatsheet