The alert box is a system object, and not subject to CSS. To do this style of thing you would need to create an HTML element and mimic the alert() functionality. The jQuery UI Dialogue does a lot of the work for you, working basically as I have described: Link.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>jQuery UI Dialog - Default functionality</title>
<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$( function() {
$( "#dialog" ).dialog();
} );
</script>
</head>
<body>
<div id="dialog" title="Basic dialog">
<p>This is the default dialog which is useful for displaying information. The dialog window can be moved, resized and closed with the 'x' icon.</p>
</div>
</body>
</html>
Answer from PCasagrande on Stack OverflowVideos
The alert box is a system object, and not subject to CSS. To do this style of thing you would need to create an HTML element and mimic the alert() functionality. The jQuery UI Dialogue does a lot of the work for you, working basically as I have described: Link.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>jQuery UI Dialog - Default functionality</title>
<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$( function() {
$( "#dialog" ).dialog();
} );
</script>
</head>
<body>
<div id="dialog" title="Basic dialog">
<p>This is the default dialog which is useful for displaying information. The dialog window can be moved, resized and closed with the 'x' icon.</p>
</div>
</body>
</html>
I use SweetAlert, It's Awesome, You will get lots of customization option as well as all callbacks

swal("Here's a message!", "It's pretty, isn't it?");

I think that the problem is that you are not telling nodeJS where your statics files are. For me, the simplest way is to set the server with Express
$ npm install express
And then setting up the server and where your static directory is:
var express = require('express');
var app = express();
//setting middleware
app.use(express.static(__dirname + 'public')); //Serves resources from public folder
var server = app.listen(5000);
There are other ways to doit using Native NodeJS, here are some resources:
Nodejs.org - How to serve static files
Also, you can write the script directly in your html file:
<!DOCTYPE html>
<html>
<head>
<script>
function showBox(){
alert("this is alert box");
}
</script>
</head>
<body onload="showBox()">
<h1>Heading</h1>
<p>Paragraph</p>
</body>
</html>
Your server is only ever returning index.html, no matter what path is requested. So, your watch.js is never loaded. The contents of index.html are returned instead of watch.js.
Either handle the other paths in your server code, or use something like Express.static, which does this for you.