When you first create a new Bootstrap document, DW has assets in a TEMP folder until you save & name your document. Assuming your site is properly defined, DW will then resolve the path and put copies of Bootstrap & jQuery in your local site's assets folder.
DO NOT edit Bootstrap files. Always use a separate external stylesheet for your custom overrides. Your custom stylesheet must appear below the Bootstrap.css file in your code.
Nancy
Copy & paste this code into a new, blank document and save it. This is what it looks like with the Darkly Theme. Other Bootstrap Themes are available from MaxCDN. Bootswatch · BootstrapCDN by MaxCDN

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Bootstrap 3.3.7 Starter</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<!-- Latest compiled and minified Bootstrap CSS, Darkly Theme-->
<link href="https://maxcdn.bootstrapcdn.com/bootswatch/3.3.7/darkly/bootstrap.min.css" rel="stylesheet" integrity="sha384-S7YMK1xjUjSpEnF4P8hPUcgjXYLZKK3fQW1j5ObLSl787II9p8RO9XUGehRmKsxd" crossorigin="anonymous">
<!--custom CSS-->
<style>
body {padding-top: 75px}
</style>
</head>
<body>
<!--begin top nav bar-->
<nav class="navbar navbar-default navbar-fixed-top" role="navigation">
<div class="container-fluid">
<div class="navbar-header"> <a class="navbar-brand" href="#">XYZ Website</a>
<button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#myNavbar">
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
</div>
<div class="collapse navbar-collapse" id="myNavbar">
<ul class="nav navbar-nav navbar-left">
<li class="active"><a href="#">HOME</a></li>
<li><a href="#">Darkly Theme</a></li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li class="dropdown"> <a class="dropdown-toggle" data-toggle="dropdown" href="#">Dropdown <span class="caret"></span></a>
<ul class="dropdown-menu">
<li><a href="#">submenu 1-1</a></li>
<li><a href="#">submenu 1-2</a></li>
<li><a href="#">submenu 1-3</a></li>
</ul>
</li>
<li><a href="#">MENU 4</a> </li>
<li><a href="#">MENU 5</a> </li>
<li><a href="#">MENU 6</a></li>
</ul>
</div>
</div>
<!--end top nav-->
</nav>
<div class="container">
<div class="row">
<div class="jumbotron">
<h1>Hello, world!</h1>
<p>This is a simple hero unit, a simple jumbotron-style component for calling extra attention to featured content or information.</p>
<p><a class="btn btn-primary btn-lg" href="#" role="button">Learn more</a>
</p>
</div>
<!--/row--></div>
<hr>
<div class="row">
<footer class="text-center"><small>© 2017 XYZ Website. All rights reserved</small></footer>
<!--/row--></div>
<!--/container--></div>
<!--latest jQuery-->
<script src="https://code.jquery.com/jquery-1.12.2.min.js" integrity="sha256-lZFHibXzMHo3GGeehn1hudTAP3Sc0uKXBXAzHX1sjtk=" crossorigin="anonymous">
</script>
<!--Bootstrap-->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
</body>
</html>