Unless you're an expert coder, DO NOT OPEN Bootstrap or jQuery files. Never, never, never open them. You should be using a separate external CSS file for your custom styles.
Copy and paste this code into a new, blank document. You can't edit these Bootstrap and jQuery files because they are hosted on CDNs.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Bootstrap</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<!-- Latest Bootstrap 3 CSS-->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
<!--CUSTOM CSS GOES HERE-->
<link rel="stylesheet" href="YOUR_STYLEHSEET.css">
</head>
<body class="container">
<div class="row">
<div class="col-md-10 center-block">
CONTENT GOES HERE....
</div>
</div>
<!--latest jQuery 3 JS-->
<script src="https://code.jquery.com/jquery-3.2.1.min.js" integrity="sha256-hwg4gsxgFZhOsEEamdOYGBf13FyQuiTwlAQgxVSNgt4=" crossorigin="anonymous"></script>
<!--Latest Bootstrap 3 JS-->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
</body>
</html>