In the beginning, I used kognise's water.css [1], so most of the smart decisions (background/text color, margins, line spacing I think) probably come from there. Since then it's been some amount of little adjustments. The font is by Jean François Porchez, called Le Monde Livre Classic [2].
I draft in Obsidian [3] and build the site with a couple python scripts and KaTeX.
cgadski: what did you use to make it?