src/LessCompiler.php
public function combineFiles($combinedFileName, $files, $options = array())
    {
        $new_files = [];

        foreach ($files as $file) {
            $new_files[] = $this->processLessFile($file);
        }

        return parent::combineFiles($combinedFileName, $new_files, $options);
    }
public function combineFiles($combinedFileName, $files, $options = array())
    {
        $new_files = [];

        foreach ($files as $file) {
            $new_files[] = $this->processLessFile($file);
        }

        return parent::combineFiles($combinedFileName, $new_files, $options);
    }
Process any less files and return new filenames
@See Requirements_Backend->combineFiles() for options