Less language is an extension of css and less4j compiles it into regular css. Less adds several dynamic features into css: variables, expressions, nested rules, and so on. Less was designed to be compatible with css and any correct css file is also correct less file.
https://github.com/SomMeri/less4j/issues/283#issuecomment-114384395
less.js supports appender inside top level rulesets e.g. this should work: