This site uses ikiwiki, a powerful open-source wiki and blog software package which uses Markdown formatting and a revision-control platform such as git to store flat files.
It is hosted by Branchable, an ikiwiki-specific hosting company run by the primary author of ikiwiki, legendary open-source developer Joey Hess. I find the prices very reasonable and the service excellent.
The look of the site is based on ikiwiki's blueview
theme, with a
custom banner and a little extra CSS. The banner image is from an
underground map of a mine in Silver City, Nevada: "Longitudinal
Section on Ledge, Northern Belle Mine," surveyed January 1876 by
Chas. F. Hoffman (scale 30 feet to an inch; squares 20 feet), Document
50180313, National Mine Map Repository, Office of Surface Mining and
Reclamation.