var sheets = document.styleSheets; // 返回一个StyleSheetList数组

/*
返回： 

StyleSheetList {0: CSSStyleSheet, 1: CSSStyleSheet, 2: CSSStyleSheet, 3: CSSStyleSheet, 4: CSSStyleSheet, 5: CSSStyleSheet, 6: CSSStyleSheet, 7: CSSStyleSheet, 8: CSSStyleSheet, 9: CSSStyleSheet, 10: CSSStyleSheet, 11: CSSStyleSheet, 12: CSSStyleSheet, 13: CSSStyleSheet, 14: CSSStyleSheet, 15: CSSStyleSheet, length: 16, item: function}
*/

// 找到你想要修改的样式表
var sheet = document.styleSheets[0];

// Get info about the first stylesheet
console.log(document.styleSheets[0]);

/*
返回结果： 

CSSStyleSheet
 cssRules: CSSRuleList
 disabled: false
 href: "http://davidwalsh.name/somesheet.css"
 media: MediaList
 ownerNode: link
 ownerRule: null
 parentStyleSheet: null
 rules: CSSRuleList
 title: null
 type: "text/css"
*/

// Get the media type
console.log(document.styleSheets[0].media.mediaText)
/*
Returns:
 "all" or "print" or whichever media is used for this stylesheet
*/

var sheet = (function() {
 // Create the <style> tag
 var style = document.createElement("style");

 // 如果你愿意的话，可以添加media属性 (或 media query) 
 // style.setAttribute("media", "screen")
 // style.setAttribute("media", "@media only screen and (max-width : 1024px)")

 // WebKit 补丁 
 style.appendChild(document.createTextNode(""));

 // Add the <style> element to the page
 document.head.appendChild(style);

 return style.sheet;
})();

function addCSSRule(sheet, selector, rules, index) {
 if(sheet.insertRule) {
 sheet.insertRule(selector + "{" + rules + "}", index);
 }
 else {
 sheet.addRule(selector, rules, index);
 }
}

// Use it!
addCSSRule(document.styleSheets[0], "header", "float: left");