This site is powered by a Hugo theme called lekh which is a Hugo clone(almost) of https://www.vegardstikbakke.com/
Lekh is my first take at designing a Hugo theme, and apart from the tiny bit of templating specific to Hugo in Golang, the rest is copied as it is from Vegard’s site. Also the dark mode is taken as it is from https://www.gwern.net/