...A custom importer allows extension of the LibSass engine in both a synchronous and asynchronous manner. In both cases, the goal is to either return or call done() with an object literal. When returning or calling done() with { file: "String" }, the new file path will be assumed for the @import. It's recommended to be mindful of the value of prev in instances where relative path resolution may be required. When returning or calling done() with { contents: "String" }, the string value will be used as if the file was read in through an external source.