A simple smooth scrolling using 100% vanilla JavaScript, and it's only 3kb!
// index.html
<html>
<head>
<link rel="stylesheet" href="./css/SmoothScroll.css">
<link rel="stylesheet" href="./css/content.css"> // Your content CSS file
</head>
<body>
<div class="viewport">
<div class="container">
// Content
</div>
</div>
<script src="./js/SmoothScroll.min.js"></script>
<script src="./js/script.js"></script>
</body>
</html>