如何修改浏览器默认的alert样式？

window.alert = function(str) 
{ 
var shield = document.createElement("DIV"); 
shield.id = "shield"; 
shield.style.position = "absolute"; 
shield.style.left = "0px"; 
shield.style.top = "0px"; 
shield.style.width = "100%"; 
shield.style.height = document.body.scrollHeight+"px"; 
//弹出对话框时的背景颜色 
shield.style.background = "#fff"; 
shield.style.textAlign = "center"; 
shield.style.zIndex = "25"; 
//背景透明 IE有效 
//shield.style.filter = "alpha(opacity=0)"; 
var alertFram = document.createElement("DIV"); 
alertFram.id="alertFram"; 
alertFram.style.position = "absolute"; 
alertFram.style.left = "50%"; 
alertFram.style.top = "50%"; 
alertFram.style.marginLeft = "-225px"; 
alertFram.style.marginTop = "-75px"; 
alertFram.style.width = "450px"; 
alertFram.style.height = "150px"; 
alertFram.style.background = "#ff0000"; 
alertFram.style.textAlign = "center"; 
alertFram.style.lineHeight = "150px"; 
alertFram.style.zIndex = "300"; 
strHtml = "<ul style="list-style:none;margin:0px;padding:0px;100%">
"; 
strHtml += " <li style="background:#DD828D;text-align:left;padding-left:20px;font-size:14px;font-weight:bold;height:25px;line-height:25px;border:1px solid #F9CADE;">[升级提示]</li>
"; 
strHtml += " <li style="background:#fff;text-align:center;font-size:12px;height:120px;line-height:120px;border-left:1px solid #F9CADE;border-right:1px solid #F9CADE;">"+str+"</li>
"; 
strHtml += " <li style="background:#FDEEF4;text-align:center;font-weight:bold;height:25px;line-height:25px; border:1px solid #F9CADE;"><input type="button" value=" 升 &nbsp; 学 " onclick="doOk()" /></li>
"; 
strHtml += "</ul>
"; 
alertFram.innerHTML = strHtml; 
document.body.appendChild(alertFram); 
document.body.appendChild(shield); 
//var ad = setInterval("doAlpha()",5); 
this.doOk = function(){ 
alertFram.style.display = "none"; 
shield.style.display = "none"; 
window.location.href="../store.html?store_id=1";
} 
alertFram.focus(); 
document.body.onselectstart = function(){return false;}; 
}