Yet another one of these. This is the template for my course-specific websites.
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.