Offtop: what kind of static website generator did he use? Looks clean, looking for something similar for my usage.
The "clean look" you refer to is mostly thanks to custom CSS though - nothing specifically related to Hakyll.
If you're thinking about building a similarly-styled blog or personal site, you could start by studying the styles this author used on their site: https://github.com/barrucadu/barrucadu.co.uk/tree/master/css