code: public function start($basePath = null, array $requiredFields = [], $exceptionOnInvalid = false)
      {
          $this->setThemeFolder($basePath);
          $this->exceptionOnInvalid = $exceptionOnInvalid;
  
          //Look for theme.yml and theme.yaml
          $this->find('theme.yml', $requiredFields);
          $this->find('theme.yaml', $requiredFields);
  
          return new ThemeCollection($this->themes, $this->themesFolder, $requiredFields, $exceptionOnInvalid);
      }
docs: @param string|null $basePath
  @param array       $requiredFields
  @param bool     $exceptionOnInvalid
  
  @return ThemeCollection