Duplicate and modify builtin theme

If I was basically happy with one of the builtin themes but wanted to tweak a couple of things, is there a way to duplicate it and work on the copy?

Yes, you just make a copy of theme directory, and you edit the files with any text editor. The exact location depends on the operating system, some documentation is here: https://github.com/Foundry376/Mailspring-Theme-Starter

that’s for building a theme based on the starter theme. I asked about using one of the builtin themes as a start point.