Enqueuing styles from manifest.json

I’ve added another library as a dependency to manifest.json:

"bootstrap-material-design.css": {
  "bower": ["bootstrap-material-design"]

bootstrap-material-design.css is compiled to dist/styles as expected.

However, the file is not automatically enqueued in the site’s head similar to how main.css and main.js are.

I can see that class JsonManifest is responsible for managing Sage’s assets, but how can I ‘tell’ it to also enqueue bootstrap-material-design.css ?

That’s not possible. You have to manually add new enqueues for additional stylesheets/scripts.