JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    Teste 1
</div>
<div class="teste">
    Teste 2
</div>
<div class="teste">
    Teste 3
</div>
<div class="teste">
    Teste 4
</div>

CSS

.classe {
    background-color:#F00;
    border:solid 4px #33CCCC;   
}

JavaScript

var primeiroTeste = document.querySelector('.teste');
primeiroTeste.classList.add('classe');