Demo

script.js
style.css
index.html
setup.css

let highlightClass = 'highlight'; // Set up variables.
let textBlock = document.querySelector('section'); // Any selector.
let switchButton = document.querySelector('#example');

switchButton.onclick = () => { // Attach the event.
	textBlock.classList.toggle(highlightClass); // Toggle the class!
};

section { /* Initial state, without the class. */
	background-color: gold;
	padding: calc(var(--base) / 2);
	transition: all 1s ease-in-out; /* Transition everything. */
	will-change: transform; /* Improves the aliasing! */
}

section.highlight { /* When the class is applied with JS! */
	background-color: aquamarine;
	transform: rotate(5deg);
}

<!DOCTYPE html>
<html>
	<head>
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<link href="/assets/styles/reset.css" rel="stylesheet">
		<link href="setup.css" rel="stylesheet">
		<link href="style.css" rel="stylesheet">
		<script defer src="script.js"></script>
	</head>
	<body>
		<section>
			<p>I’ve just added some text here to have another element, and also added some CSS to style it a bit—nothing too fancy.</p>
		</section>
		<button id="example">Click here!</button>
	</body>
</html>

html, body { height: 100%; }

body {
	--base: 20px;

	align-items: center;
	display: flex;
	flex-direction: column;
	font-family: sans-serif;
	justify-content: center;
	padding: var(--base);
	row-gap: var(--base);
}

button {
	background-color: deepskyblue;
	border-radius: calc(var(--base) / 2);
	cursor: pointer; /* Show the Mickey-Mouse hand! */
	padding: calc(var(--base) / 2);
}