Manual:$wgResourceLoaderLESSImportPaths

Details
Default import paths for LESS modules. LESS files referenced in @import 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 $wgResourceLoaderLESSImportPaths. The import path includes the path of the currently compiling LESS file, which allows each extension to freely import files from its own tree.