重写alert弹出窗口样式

<script>

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;width:100%\">\n";
     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;\">[自定义提示]</li>\n";
     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;\">"+"<strong>"+str+"</strong>"+"</li>\n";
     strHtml += " <li style=\"background:#FDEEF4;text-align:center;font-weight:bold;height:25px;line-height:25px; border:1px solid #F9CADE;\"><input type=\"button\" value=\"确 定\" onclick=\"doOk()\" /></li>\n";
     strHtml += "</ul>\n";
     alertFram.innerHTML = strHtml;
     document.body.appendChild(alertFram);
     document.body.appendChild(shield);
     this.doOk = function(){
         alertFram.style.display = "none";
         shield.style.display = "none";
     }
     alertFram.focus();
     document.body.onselectstart = function(){return false;};
    }

alert(‘513795‘);//这里的 513795 相当于str的值
    </script>

重写alert弹出窗口样式