I used Asciidoctor. It's mostly the default theme, but I tweaked a few things about it.

If you're interested in the details, I wrote a blog post that goes into it a bit more: https://www.kevinwmcconnell.com/writing/how-i-wrote-my-book

That post also has a link to a repo with all the settings/theme/etc, so anyone can take it and use it as a starting point for their own book.