This is a Hugo theme for my course websites.
This theme is a very lightly modified version of Andrew Heiss's ath-tufte-hugo theme. He says of it:
Tufte's CSS is relatively basic and doesn't deal with headers and footers and other basic website elements. The Tufte Jekyll theme has all these, but it's written for Jekyll (and it's written in Sass).
So, I went to the demo page, copied the compiled CSS file there, and used that as my base CSS file. It probably still has a lot of Jekyll-specific things that I don't use, but ¯\_(ツ)_/¯.
And then I pilfered from John Martin's excellent modifications and made my own.