How to include a third-party CSS file in my custom plugin?

[cool]

A third party library has a CSS file (not a .less file). How can I include it in my plugin. Is plugin.json the right place? If yes, under what subheading shall I place it?

[PitaJ]

You can use import in one of your less files like this:

@import (inline) './thing.css';

Or you can add an entry to staticDirs and add it to the header:
https://github.com/NodeBB/nodebb-plugin-emoji/blob/master/lib/index.ts#L80

Or you can add it under the less array.

[julian]

Please check out the theming documentation