It might be Jekyll according to https://robertheaton.com/2014/07/26/lessons-from-a-surprisin...

Looking at the page source, it doesn't appear to be Wordpress; none of the usual cruft that WP scatters through the source (though it could have been processed via a filter).

Looking at the request/response headers in Firefox's web developer tools, it's gone through Cloudflare, but there's also a x-github header, so maybe some custom tooling that's pushed content to github?

It's jekyll: https://github.com/robert/robert.github.com

Nowadays, many sites are built using a static site generator. Have a look at Hugo, Jekyll, Gatsby, etc.

Usually you have plenty of themes to choose from. I use Hugo and can tell you there are many themes similar to the one in this website.