const body = document.querySelector("body");
body.style.background = "url(https://i.imgur.com/fOfpsiC.png) no-repeat";
/* -9999px -9999px makes the image "disappear" not sure what you are trying to do with it? */
/* body.style.background = "url(https://i.imgur.com/fOfpsiC.png) no-repeat -9999px -9999px"; */
body.style.content = "";
body.style.position = "absolute";
Not really possible, as @lasjorg says — the way the guy in the linked article gets around it is insane. You are using JS, you can insert any real element into the DOM, you are not restricted as you are in CSS. Also, syntax for pseudo elements is :: not :