Post your plugin.json maybe it has a syntax error. Also did you put your theme in node_modules and activate it from the ACP?
Doc mistakes the nbbpm property
The documentation states
nbbpm.compatibilityas a property within plugin.json rather than package.json.
I guess this is a bug? Or is the property allowed either place these days?
You're right. We need to have a look at that page, it's starting to get out of date