<link href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.0/css/bootstrap.min.css" rel="stylesheet" id="bootstrap-css">
<script src="//maxcdn.bootstrapcdn.com/bootstrap/3.3.0/js/bootstrap.min.js"></script>
<script src="//code.jquery.com/jquery-1.11.1.min.js"></script>
<!------ Include the above in your HEAD tag ---------->
<div class="container">
<div class="row">
<div class="header">the header</div>
</div>
</div>
$(window).on("scroll", function() {
if($(window).scrollTop() > 50) {
$(".header").addClass("active");
} else {
//remove the background property so it comes transparent again (defined in your css)
$(".header").removeClass("active");
}
});