code: def set_style_sheet(self, stylesheet):
          
          self._document.setDefaultStyleSheet(stylesheet)
          self._style = None
          self._clear_caches()
docs: Sets a CSS stylesheet. The classes in the stylesheet should
          correspond to those generated by:
              pygmentize -S <style> -f html
          Note that 'set_style' and 'set_style_sheet' completely override each
          other, i.e. they cannot be used in conjunction.