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() {
    for (var ssIdx = 0; ssIdx < document.styleSheets.length; ssIdx++) {
        var ss = document.styleSheets[ssIdx];
        var rules = ss.cssRules || ss.rules;
        var h1Rule = null;
        for (var ruleIdx = 0; ruleIdx < rules.length; ruleIdx++) {
            var rule = rules[ruleIdx];
            if (rule.selectorText == "h1") {
                rule.style.border = "1px solid blue";
                return;
            }
        }
    }
}
$(function () {
    $("a").click(function (e) {
        e.preventDefault();
        changeRule();
    });
});