You have a directory (colour-scheme/) containing .less (each defining a different colour scheme). For each of those files (call it scheme-x.less), compile it with the "core" .less file(s) (stored elsewhere) to create style-x.css.
Preferably, by adding a .less file to colour-scheme/ directory and running a grunt task, the appropriate style-?.css should be created. That is, no editing of Gruntfile.js is required.
Directory layout is below.