JSFiddle - React, Tailwind, and code Playground

HTML

<style id="style">body{background: cornflowerblue;}</style>

<button id="injectButton">Inject Styles</button>

CSS

body { 
    transition: 1s;
    -webkit-transition; 1s;
    -moz-transition: 1s;
}

JavaScript

function injectStyles() {   
    var style = document.getElementById("style");
    var newStyles = "body{background:#aaa}";
    style.appendChild(document.createTextNode(newStyles));
    alert("injected styles");
}
    
var button = document.getElementById("injectButton");
button.addEventListener("click", injectStyles, false);