How to properly use less with play 2.8.x?

Hi there, I’m trying to use less with play following this doc.

Reading the doc seems to me that i just need a main.less file under app/assets/stylesheets and that will be compiled to a main.css. But for me, the build process is just copying exactly the main.less file to target/web/public/main/stylesheets without doing any processing.
Maybe I’m missing some configuration, I have a new play project and for now i just created the app/assets folder.