Maestro styling system is based on LESS, and all custom code is interpreted as LESS. LESS is a superset of CSS, which means that all normal CSS is valid within a LESS context too.
SASS is not supported. Well technically, if you used the conventional web dev workflow of compiling SASS in a preprocess stage, along with the new SCM features coming in 17.10, you could use the Ruby-based preprocessor to compile SASS assets and then push the resulting CSS to Maestro. Certainly not as smooth and well integrated as the built-in LESS support though.
We use Less.