code: function (sheet, selector, rule) {
              var index;
              if (sheet.insertRule) {
                  return sheet.insertRule(selector + '{' + rule + '}', sheet.cssRules.length);
              } else {
                  index = sheet.addRule(selector, rule, sheet.rules.length);
                  if (index < 0) {
                      index = sheet.rules.length - 1;
                  }
                  return index;
              }
          }
docs: Append a CSS rule to the given stylesheet
  @param   {CSSStyleSheet} sheet The stylesheet object
  @param   {string} selector     The selector
  @param   {string} rule         The rule
  @returns {int}                 The index of the new rule