Hi poonam,
Override the javascript window.alert() function.
Check this example.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style type="text/css">
#modalContainer
{
background-color: rgba(0, 0, 0, 0.3);
position: absolute;
top: 0;
width: 100%;
height: 100% !important;
left: 0px;
z-index: 10000;
background-image: url(test.png); /* required by MSIE to prevent actions on lower z-index elements */
}
#alertBox
{
position: relative;
width: 250px;
min-height: 50px;
margin-top: 50px;
border: 1px solid #fff;
background-color: #fff;
background-repeat: no-repeat;
}
#modalContainer > #alertBox
{
position: relative;
}
#alertBox h1
{
margin: 0;
font: bold 1em Raleway,arial;
background-color: #f97352;
color: #FFF;
border-bottom: 1px solid #f97352;
padding: 10px 0 10px 5px;
text-align: center;
}
#alertBox p
{
height: 50px;
padding-left: 5px;
padding-top: 30px;
text-align: center;
vertical-align: middle;
}
#alertBox #closeBtn
{
display: block;
position: relative;
margin: 30px auto 10px auto;
padding: 7px;
border: 0 none;
width: 70px;
text-transform: uppercase;
text-align: center;
color: #FFF;
background-color: #f97352;
border-radius: 0px;
text-decoration: none;
outline: 0 !important;
}
</style>
<script type="text/javascript">
if (document.getElementById) {
window.alert = function (txt) {
CreateCustomAlert(txt, "Cancel", "Custom Alert");
}
}
function CreateCustomAlert(txt, ALERT_BUTTON_TEXT, ALERT_TITLE) {
d = document;
if (d.getElementById("modalContainer")) {
return;
}
mObj = d.getElementsByTagName("body")[0].appendChild(d.createElement("div"));
mObj.id = "modalContainer";
mObj.style.height = d.documentElement.scrollHeight + "px";
alertObj = mObj.appendChild(d.createElement("div"));
alertObj.id = "alertBox";
if (d.all && !window.opera) {
alertObj.style.top = document.documentElement.scrollTop + "px";
}
alertObj.style.left = (d.documentElement.scrollWidth - alertObj.offsetWidth) / 2 + "px";
alertObj.style.visiblity = "visible";
h1 = alertObj.appendChild(d.createElement("h1"));
h1.appendChild(d.createTextNode(ALERT_TITLE));
msg = alertObj.appendChild(d.createElement("p"));
msg.innerHTML = txt;
btn = alertObj.appendChild(d.createElement("a"));
btn.id = "closeBtn";
btn.appendChild(d.createTextNode(ALERT_BUTTON_TEXT));
btn.href = "#";
btn.focus();
btn.onclick = function () {
document.getElementsByTagName("body")[0].removeChild(document.getElementById("modalContainer"));
return false;
}
alertObj.style.display = "block";
}
window.onload = function () {
alert('Thank you!');
};
</script>
</head>
<body>
<div>
</div>
</body>
</html>
Demo