code: def add_user_css(self, css_list):
          
          if isinstance(css_list, string_types):
              css_list = [css_list]
  
          for css_path in css_list:
              if css_path and css_path not in self.user_css:
                  if not os.path.exists(css_path):
                      raise IOError('%s user css file not found' % (css_path,))
                  with codecs.open(css_path, encoding=self.encoding) as css_file:
                      self.user_css.append({
                          'path_url': utils.get_path_url(css_path,
                                                         self.relative),
                          'contents': css_file.read(),
                      })
docs: Adds supplementary user css files to the presentation. The
              ``css_list`` arg can be either a ``list`` or a string.