Adds a dark mode to ZyBooks
TL;DR I was tired of having my eyes assaulted by the blinding light of a thousand suns when all I wanted to do was learn code.
This adds a dark theme to ZyBooks learn-to-do-stuff pages as well as highlighting and setting monotype fonts for elements that contain (or are likely to contain) code.
This isn't a complete dark theme, there will be missing elements I haven't encountered yet and there are some elements (especially colored code and animations) that don't have specific enough identifiers to select with CSS and/or are colored by an element style rather than a class and would be [effort] to change.
