JSFiddle - React, Tailwind, and code Playground

HTML

<body class="fifty">
    Here
</body>

CSS

body{ height:5000px; }
.fifty { background:rgba(25,20,25,.5) }
.ninty {  background:rgba(25,20,25,.9) }

JavaScript

$(document).ready(function(){       
    var scroll_pos = 0;
    $(document).scroll(function() { 
        scroll_pos = $(this).scrollTop();
        if(scroll_pos > 210) {
            $("body").removeClass().addClass('ninty');
        } else {
            $("body").removeClass('ninty').addClass('fifty');
        }
    });
});