<!DOCTYPE html>
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.2/jquery.min.js"></script>
<script>
$(document).ready(function(){
$(window).scroll(function() {
if ($(document).scrollTop() > 50) {
$("p").addClass("test");
} else {
$("p").removeClass("test");
}
});
});
</script>
<style>
.test {
background-color: yellow;
}
</style>
</head>
<body style="height:1550px;">
<p>Scroll down this page.</p>
<p style="position:fixed;">When you have scrolled 50 pixels from the top, add the class "test" (yellow background color) to the paragraph. Scroll up again to remove the class.</p>
</body>
</html>