From this link [0], it appears that the site uses Netlify. I'd love for the OP to point us in the direction of the theme and/or setup details as I am fairly new to Netlify
It is just static HTML and CSS files, with sprinkles of JS when necessary. I don't like using third-party frameworks or themes because they tend to get in my way. I designed it all from scratch.
I've been experimenting with functional (atomic) CSS lately and built a small framework that ended up using in Hexadecimal (if you peek at the source code, you can see weird classes like m-0, p-0, etc..). I liked it, but one should be careful in not overusing it, otherwise, it is an unmaintainable mess, yet again.
Below, I linked my Netlify config [0]. My static files live in the src/ folder, so you should probably change that if you have them in a different folder. If you don't know what Content-Security-Policy (CSP) or Strict-Transport-Security (HSTS) headers do, leave them out. It is easy to shoot yourself in the foot if you don't know what you're doing.
Very clean and simple, but not trivial or bare. Really good job, jmstfv.