JSFiddle - React, Tailwind, and code Playground

HTML

<div id="baslik">ekipcalismasi.blogspot.com</div>
<br>
<button id="css">Değiştir</button>

CSS

div
{
    background-color:#DDD;
}

.kirmizi
{
	background-color:#F00;
	width:100px;
	height:100px;
}

.normal
{
    
}

JavaScript

$(document).ready(function(e) {
    $("#css").click(function(e){
		$("#baslik").addClass("kirmizi");
		});
});