JSFiddle - React, Tailwind, and code Playground

HTML

<p>Testing</p>

CSS

body {
    color: red;
}

JavaScript

var s = document.styleSheets[1];

console.log(s);

function changeStylesheetRule(stylesheet, selector, property, value) {
	selector = selector.toLowerCase();
	property = property.toLowerCase();
	value = value.toLowerCase();

	for(var i = 0; i < s.cssRules.length; i++) {
		var rule = s.cssRules[i];
		if(rule.selectorText === selector) {
			rule.style[property] = value;
			return;
		}
	}
  
	stylesheet.insertRule(selector + " { " + property + ": " + value + "; }", 0);
}

changeStylesheetRule(s, "body", "color", "rebeccapurple");
changeStylesheetRule(s, "p", "border", "1px solid rebeccapurple");

console.log(s);