JSFiddle - React, Tailwind, and code Playground

by Nikolay Vasilchuk

HTML

<div...

CSS

.toolbar {font-weight: bold; position: fixed; background: red; font-size: 20px}
.toolbar2, #yourElement {font-style: italic; position: fixed; background: red}

JavaScript

var div = document.getElementById('yourElement');
window.onscroll = function() {
    if (window.pageYOffset > 100) {
        div.classList.add('toolbar');
    }
    else if (window.pageYOffset < 100) {
        div.classList.remove('toolbar');
    }
};