请问 :javascript 如何动态改变外部样式链接
比如在一个HTML页面有这个外部样式链接: 
  <link   type= "text/css "   rel= "stylesheet "   href= "css/main.css "   />  
 现在我想通过javascript动态更换成另一个: 
  <link   type= "text/css "   rel= "stylesheet "   href= "css/main2.css "   />    
 该   如何实现?? 
 请高手赐教!!谢谢了~~
------解决方案-------------------- <script>  
 if(){ 
   css= 'main.css '; 
 }else{ 
   css= 'main2.css '; 
 } 
 document.write ' <link type= "text/css " rel= "stylesheet " href= "css/ '+css+ ' " />  
  ' 
  </script>    
------解决方案-------------------- <link type= "text/css " id= "xx " rel= "stylesheet " href= "css/main.css " />    
 xx.setAttribute( "href ", "css/main2.css ") 
------解决方案--------------------另外一种方式   
  <html>  
   <head>  
    <link href= "stylesheet1.css " type= "text/css " rel= "stylesheet " id= "mycss ">  
    <script language= "javascript ">  
   var turnback=false; 
   function setCSS() 
   { 
    mycss.styleSheet.cssText= " "; 
    mycss.styleSheet.addImport(turnback? "stylesheet1.css ": "stylesheet2.css "); 
    turnback=!turnback; 
   } 
    </script>  
   </head>  
   <body>  
    <button onclick= "setCSS() " type= "button "> 切换背景颜色 </button>  
   </body>  
  </html>      
 StyleSheet1.CSS样式代码: BODY {  background-color: Black; }   
 StyleSheet2.CSS样式代码:BODY {  background-color: maroon; }