The theme doesn't build under the latest Node

I tried to build the theme as part of a local copy of `hmnd.md`. I had the latest version of Node installed (16) and the `npm install` command failed.
There were a lot of errors generated (log file was 12000 lines long) so it wasn't easy to figure out what is borken.
In the end I installed `nvm` and tried older versions.
v12, and v10 (maybe) did not build either. I jumped back to v8 based on the date of the last significant update to the theme and that worked.