scrollbar-parts-opacity.html

<!DOCTYPE html>

<html>
<head>
    <style>
        body {
            margin: 0;
        }
        ::-webkit-scrollbar {
            width: 60px;
            height: 60px;
            background-color: green;
            opacity: 0.75;
        }

        ::-webkit-scrollbar-thumb {
            background-color: navy;
            opacity: 0.5;
        }
        
        .overflow {
            position: absolute;
            height: 400px;
            width: 400px;
            overflow: scroll;
        }
        .contents {
            height: 800px;
            width: 800px;
        }
        
        .backdrop {
            position: absolute;
            top: 100px;
            left: 100px;
            height: 400px;
            width: 400px;
            background-color: gray;
        }
        
        .vertical {
            width: 150px;
        }

        .horizontal {
            height: 150px;
        }
    </style>
</head>
<body>

    <div class="backdrop vertical"></div>
    <div class="backdrop horizontal"></div>
    <div class="composited overflow">
        <div class="contents"></div>
    </div>
</body>
</html>