adds dark mode

[ivanchubb]

adds dark mode, quarto will just add a slider at the top right of the page, should still default to the first theme "light" which is your native litera and html/custom.scss. Additionally, you could add your custom.scss file to the dark mode theme, but the colors may contrast with darkly... or not. I wasn't able to build the full site without installing 20+ dependencies so i was only able to test on the "index.html" main page. It looked good enough without the custom.scss so i left it out. if you are able to build the full site locally then you'll be able to experiment more thoroughly than I.