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.