"Dark Mode" Build

[JDSherbert]

Add a Dark Mode toggle/build for the website before @Sommos needs to get major eye surgery.

Note that for a css solution, some of it is based on the theme itself:
https://just-the-docs.github.io/just-the-docs-template/

Consider using this project as a reference:
https://github.com/andrewhwanpark/dark-poole