code: def _get_css_files(cls, extra_files):
          
          packager = Packager()
          css_packages = getattr(cls, 'css_packages', {})
  
          return dict(
              (media_target,
               cls._get_media_files(packager=packager,
                                    media_packages=media_packages,
                                    media_type='css',
                                    extra_files=extra_files.get(media_target,
                                                                [])))
              for media_target, media_packages in six.iteritems(css_packages)
          )
docs: Return all CSS files from the Media class.
  
          Args:
              extra_files (dict):
                  The contents of the Media class's original :py:attr:`css`
                  attribute, if one was provided.
  
          Returns:
              dict:
              The CSS media types and files to return for the :py:attr:`css`
              attribute.