Thanks Graham. The uuidgenerator worked great. That was the missing key in the theme files.
If you rename the file to themeName.plist from themeName.tmTheme, you can
open it with Xcode. You should then see a tree with "Root" and under that,
various fields.
There should be a uuid field, if not you can add it.
To get the key, you just run "uuidgen" in your Terminal. If that sounds
scary, https://www.uuidgenerator.net/ will make you one.