JSFiddle - React, Tailwind, and code Playground

HTML

<header>
    <div class="btn">btn</div>
    <div class="btn-2">btn-2</div>
</header>

CSS

header {padding: 10px;}
div {display: inline-block; margin-right: 10px; padding: 10px; border: 1px solid blue; cursor: pointer;}
.hello {border: 1px solid red;}

JavaScript

$('div.btn').click(function () {
	$('header').addClass('hello');
});

$('.hello div.btn-2').on('click', function () {
	$('header').removeClass('hello');
});