JSFiddle - React, Tailwind, and code Playground

HTML

<div id="Button">PRESS ME</div>

CSS

#Button{
width: 120px;
height : 30px;
position: fixed;
top: 100px;
left:300px;
border: 1px solid black;
background-color : rgba(0,0,255,0.3);
font-size : 25px;
}

JavaScript

var Button = document.getElementById('Button');
Button.onmouseover = function(){
this.style.backgroundColor  = 'rgba(0,255,0,0.3)';
};