JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Header</h1>
<a href="#">Turn the border blue</a>

CSS

body
{
    font-family: sans-serif;
}
h1
{
    margin: 10px 0;
    padding: 0 5px;
    font-size: 2em;
    border: 3px double red;
}

JavaScript

function changeRule() {
    var ss = document.styleSheets[2];
    var rules = ss.cssRules || ss.rules;
    var h1Rule = null;
    for (var i = 0; i < rules.length; i++) {
        var rule = rules[i];
        if (/(^|,) *h1 *(,|$)/i.test(rule.selectorText)) {
            h1Rule = rule;
            break;
        }
    }
    h1Rule.style.border = "1px solid blue";
}
$(function() {
    $("a").click(function(e) {
        e.preventDefault();
        changeRule();
    });
});