Agreed! I think I've seen it before as a theme, but I'm not sure what it's called. Wish there was a CSS version available somewhere.
(Edit: thought it looks like it's not using small-caps variants for numerals, which is a shame)
It's a variation on a LaTeX style called classicthesis (http://www.miede.de/#classicthesis) based on Robert Bringhurst's Elements of Typographic Style. Unfortunately this particular version has messed up some things (notably line length), so use the original!
Beautiful as it may be idk where I'm expected to find the time to read 500+ pages of it