I couldn't get this to work until I added less to my package.json, and re-ran npm install.
I have no idea what might be/might have been going wrong, but even after switching node.js versions, repeatedly deleting out node_modules, and doing whatever it was I could, I still couldn't get this to work - I kept getting a message saying: Cannot find module 'less'.
I finally, on a whim, decided to add 'less' to my devDependencies, and then it magically worked. And I can see that 'less' is prefectly clearly added to your own devDependencies, so I don't know if this is a bug in Node, or what it could be. Still, if that's something that's expected, then maybe that could be added to the README. And if it's not - then maybe there's a bug somewhere?
I'm happy to help try and track it down, just let me know what I can do to help.
Thanks for making a great package!