JSFiddle - React, Tailwind, and code Playground

HTML

<div class=container>f</div>
<button>full site</button>

CSS

.container, .container.full {
    background-color: green;
}

@media (max-width: 400px) {
    .container {
        background-color: red;
    }
}

JavaScript

document.querySelector("button").addEventListener("click", function () {
    document.querySelector("div").classList.add("full");
});