Implement dark mode/light mode

[zediious]

Bootstrap has a data-bs-theme attribute that can be applied to the body of the HTML doc, with either dark or light as a value. This can be used to toggle between dark and light mode very easily. This will require me to restructure classes throughout all templates, to remove the many direct references to dark and text-white classes.

This will be used to provide an admin-configurable default, and to provide users with a button to change their "mode" with simple JS, and retain the setting with local storage or sessions.