This PR updates the docs syntax CSS to what I use on <markdotto.com> ahead of shipping dark mode, which requires wider usage of CSS variables. Not a huge departure, and still something I'd like to tweak over time mind you. Open to feedback here still.
/cc @XhmikosR
Cherry-picked from #35857 at abba53d4c7e44cbd749fadf201314d846f99a498 (which is why I named this branch dancing-queen lol).