How to create a "; document.body.appendChild(divNode);

Why not this?

var styleNode = document.createElement("style");
document.head.appendChild(styleNode);

Henceforward you can append CSS rules easily to the HTML code:

styleNode.innerHTML = "h1 { background: red; }\n";
styleNode.innerHTML += "h2 { background: green; }\n";

...or directly to the DOM:

styleNode.sheet.insertRule("h1 { background: red; }");
styleNode.sheet.insertRule("h2 { background: green; }");

I expect this to work everywhere except archaic browsers.

Definitely works in Chrome in year 2019.