code: def _retrieve_styles(self):
          
          if self._styles_retrieved:
              return
          self._styles_retrieved = True
  
          try:
              self.assets.retrieve_styles(url_for('asset'))
          except Exception as ex:
              if self.debug:
                  print(format_exc(), file=sys.stderr)
              else:
                  print(' * Error: could not retrieve styles:', ex,
                        file=sys.stderr)
          if self.render_inline:
              self._inline_styles()
docs: Retrieves the style URLs from the source and caches them. This
          is called before the first request is dispatched.