Easy way for creating and modifying CSS Rules in JS

cssRule.js

if (!document.styleSheets.length) document.head.appendChild(document.createElement('style'));
var sheet = document.styleSheets[document.styleSheets.length - 1];
var rules = {};
function cssRule(selector, styles) {
	var index;
	if (selector in rules) {
		index = rules[selector];
		sheet.deleteRule(index);
	} else {
		index = rules[selector] = sheet.cssRules.length;
	}

	sheet.insertRule(selector + " {" + styles + "}", index);
	// Don't support for <IE9 sheet.addRule, sorry, ha!
}

example.js

cssRule('body', 'background-color: black;')

cssRule('body', 'background-color: white;')