I have the files in the same folder, it was just working…
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>TEST</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" type="text/css" media="screen" href="style.css" />
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<!-- jQuery library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<!-- Latest compiled JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
</head>
<body>
<section>
<article>
<div class="box"></div>
</article>
</section>
</body>
</html>
body {
background: black;
}
.box {
background: white;
width: 300px;
height: 300px;
position: relative;
margin: 0 auto;
top: 200px;
}