Create a stylesheet with JS

create a stylesheet

var sheet = (function() {
  // Create the <style> tag
	var style = document.createElement("style");

	// Add a media (and/or media query) here if you'd like!
	// style.setAttribute("media", "screen")
	// style.setAttribute("media", "@media only screen and (max-width : 1024px)")

	// WebKit hack :(
	style.appendChild(document.createTextNode(""));

	// Add the <style> element to the page
	document.head.appendChild(style);

	return style.sheet;
})();

sheet.addRule("#myList li", "float: left; background: red !important;", 1);