code: public function addSource($sourcePath, $useStrict=true)
      {
          if (is_link($sourcePath)) {
              return $this->addSource(realpath($sourcePath), $useStrict);
          }
          
          if (is_dir($sourcePath)) {
              $this->sourceDirs[] = $sourcePath;
          } elseif (true === $useStrict) {
              throw new \Exception(sprintf('Path {%s} is not a readable directory', $sourcePath));
          }
          
          return $this;
      }
docs: Adds a directory to the array of sources that will
  be searched for themes to load.