This PR adds even more CSS variables to the root and various elements, including headings, links, code, kbd, and even our tables. Some of the changes here are duplicative of other efforts—e.g., dark mode's PR has some of these new border variables. I'll handle rebasing and any conflicts that arise from that. Also worth noting that there are a few new calc() functions being added to support math against CSS variables.
Also included here is a new reassigned Sass variable to shorten $variable-prefix to $prefix moving forward. I don't know why we went with something so long, but it's annoying to type out repeatedly and I'm not yet ready to implement functions or mixins to generate our CSS variables at this time.
Definitely recommend looking at these commits one-by-one during review to see what I'm after and how I'm handling things.