Add dark-mode support to v4-lab

[dpvc]

This is my PR to add dark-mode support to the lab. I know you already did that for yourself, but I needed a dark-mode label in order to test the dark-mode support in the PR I have made for MathJax and the mj-context-menu, so I thought I'd post this one as well. It is probably different from yours, so if you want to adjust this to match your setup better, then go ahead.

[dpvc]

As an aside, we probably need to do some maintenance on this repository, as there are a bunch of unmerged branches, and the package.json versions are out of date.