Manual:$wgResourceLoaderLESSImportPaths/en

Details
Default import paths for LESS modules. LESS files referenced in  statements will be looked up here first, and relative to the importing file second. To avoid collisions, it's important for the LESS files in these directories to have a common, predictable file name prefix.

Extensions need not (and should not) register paths in. The import path includes the path of the currently compiling LESS file, which allows each extension to freely import files from its own tree.