This site is hand-coded by me.
I've used Google Fonts for the typefaces. For headings it's Libre Baskerville. It's "based on the American Type Founder's Baskerville from 1941, but it has a taller x-height, wider counters and a little less contrast, that allow it to work well for reading on-screen."
For everything else it’s the lovely, IBM Plex Sans. Apparently, it's a "neutral, yet friendly Grotesque style typeface... and has excellent legibility in print, web and mobile interfaces."