scrollbar/demo.html

<head>
    <link rel="stylesheet" href="scrollbar.css">
    <script src="scrollbar.js"></script>
    
    <script>
        function toggleDesign() {
            ["dark", "light"].forEach(s => document.body.classList.toggle(s));
        }
    </script>
    
    <style>
        body.dark {
            background: #333;
        }

        body.light {
            background: #d8d8d8;
        }

        .dark #content {
            background: #aaa;
        }

        .light #content {
            background: #efefef;
        }

        .dark button {
            background: #555;
            color: #eee;
        }

        #content {
            height: 200vh;
            width:   70vw;
            margin: 1.6em auto;
            padding:  5em;
        }
    </style>
</head>

<body class="light">
    <div id="content">
        sample content
        <br>
        <button onclick="toggleDesign()">Change design (dark/light)</button>
    </div>
</body>