http://jarvis2i2vp4j4tbxjogsnqdemnte5xhzyi7hziiyzxwge3hzmh57zad.onion/notes/dark-mode
A good place to start is by separating any color rules — your background, text, links, etc. — into a different section of your CSS. Using SASS or SCSS makes this a whole lot easier with nesting but is not required; this was written with a KISS mentality.