Today's workflows assume that CSS by itself usually isn’t enough. Regardless of the fact that current specs of CSS include variables and more advanced features, possibility to use advanced functionalities, (includes, functions, mixins), and have all that be compiled in more old browser friendly css is a must for production projects.
Having that in mind Momentum has integrated Less, SCSS preprocessors for you to use.
Current limitation is that only one file is available for editing. Multiple files, imports, and folder structure for your css/Less/Scss files are currently not available.