JSFiddle - React, Tailwind, and code Playground

HTML

<div id="theDiv"></div>

CSS

#theDiv {
    height: 100px;
    background: red;
}
#theDiv:before {
    content:' ';
    display: block;
    width: 50px;
    height: 50px;
    background: green;
}

JavaScript

var styleElem = document.head.appendChild(document.createElement("style"));

styleElem.innerHTML = "#theDiv:before {background: black;}";