code: function (selectorText, css) {
              var sheet = this.sheet;
              var rulesName = this.rulesName;
              var cssRules = this.cssRules;
              var rule = cssRules[selectorText];
              var multiSelector = selectorText.split(/\s*,\s*/);
              var i;
              if (multiSelector.length > 1) {
                  for (i = 0; i < multiSelector.length - 1; i++) {
                      this.set(multiSelector[i], css);
                  }
                  return this;
              }
              if (rule) {
                  css = toCssText(css, rule.style.cssText);
                  if (css) {
                      rule.style.cssText = css;
                  } else {
                      // unset remove this rule
                      delete cssRules[selectorText];
                      for (i = sheet[rulesName].length - 1; i >= 0; i--) {
                          if (sheet[rulesName][i] === rule) {
                              deleteRule(sheet, i);
                              break;
                          }
                      }
                  }
              } else {
                  var len = sheet[rulesName].length;
                  css = toCssText(css);
                  if (css) {
                      insertRule(sheet, selectorText, css, len);
                      cssRules[selectorText] = sheet[rulesName][len];
                  }
              }
              return this;
          }
docs: Set sheet's rule by selectorText and css.
  @param {String} selectorText selector text separated by ,
  @param {Object} css style declaration object. set value to "" to unset
  
  for example:
  // set
  set('p',{color:'red'})
  // unset
  set('p',{color:''})
  @chainable