效果如下：
all code:
<!DOCTYPE html>
<html lang="zh-cn">
<head>
<meta charset="UTF-8">
<title>转盘抽奖 自定义alert弹出框样式</title>
<meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no">
<link rel="stylesheet" type="text/css" href="css/main.css">
</head>
<body>
<div class="turnplate_box">
<canvas id="myCanvas" width="300px" height="300px">抱歉！浏览器不支持。</canvas>
<canvas id="myCanvas01" width="200px" height="200px">抱歉！浏览器不支持。</canvas>
<canvas id="myCanvas03" width="200px" height="200px">抱歉！浏览器不支持。</canvas>
<canvas id="myCanvas02" width="150px" height="150px">抱歉！浏览器不支持。</canvas>
<button id="tupBtn" class="turnplatw_btn"></button>
</div>
</body>
</html>
<script src="js/jquery-1.11.3.min.js"></script>
<script src="js/main.js"></script>
<!-- 更改系统默认弹窗 -->
<script>
window.alert = function(str)
{
var shield = document.createElement("DIV");
shield.id = "shield";
shield.style.position = "absolute";
shield.style.left = "50%";
shield.style.top = "50%";
shield.style.width = "280px";
shield.style.height = "150px";
shield.style.marginLeft = "-140px";
shield.style.marginTop = "-110px";
shield.style.zIndex = "25";
var alertFram = document.createElement("DIV");
alertFram.id="alertFram";
alertFram.style.position = "absolute";
alertFram.style.width = "280px";
alertFram.style.height = "150px";
alertFram.style.left = "50%";
alertFram.style.top = "50%";
alertFram.style.marginLeft = "-140px";
alertFram.style.marginTop = "-110px";
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:#626262;text-align:left;padding-left:20px;font-size:14px;font-weight:bold;height:25px;line-height:25px;border:1px solid #F9CADE;color:white\">[中奖提醒]</li>\n";
strHtml += " <li style=\"background:#787878;text-align:center;font-size:12px;height:95px;line-height:95px;border-left:1px solid #F9CADE;border-right:1px solid #F9CADE;color:#DCC722\">"+str+"</li>\n";
strHtml += " <li style=\"background:#626262;text-align:center;font-weight:bold;height:30px;line-height:25px; border:1px solid #F9CADE;\"><input type=\"button\" value=\"确 定\" onclick=\"doOk()\" style=\"width:80px;height:20px;background:#626262;color:white;border:1px solid white;font-size:14px;line-height:20px;outline:none;margin-top: 4px\"/></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;};
}
</script>