https://github.com/bluesky-social/atproto-website
If I'm understanding this codebase right, it's a hand-rolled framework based on MDX and Next.js; the UI elements you're referring to were designed from scratch and aren't based on any existing CMS theme.
Some of it looks like tailwind css and they have a syntax highlighting addon that shows up in the homepage source as well.
some modified vercel template by the looks of it