JSFiddle - React, Tailwind, and code Playground

HTML

<button>Click Me</button>
<div id="bgr"></div>

CSS

button {
    position:relative;
    z-index:2;
}

#bgr {
    opacity:0;
    visibility:hidden;
    position:absolute; 
    top:0; 
    left:0; 
    z-index:1;
    height: 100%;
    width:100%;
    background: grey;
    -webkit-transition:all 1s;
    transition:all 1s;
}
   
 #bgr.display {
    opacity:1;
    visibility:visible;
}

JavaScript

$('button').click(function(){
    $('#bgr').toggleClass('display');
});