Add support for dark mode

This takes advantage of Bootstrap v5 which supports this out of the box. I have started with approaches from Debusine, and added support for changing the theme on the fly when the desktop preferences change.

Link: https://www.freexian.com/blog/debusine-dark-mode/ Link: https://www.xjavascript.com/blog/how-do-i-detect-dark-mode-using-javascript/#5-handling-dynamic-theme-changes