Targeting the body background-image:

body {
  background-image:
}

What would go in here?
https://jsfiddle.net/kfczgvu9/4/

const cover = document.querySelector("");

(function manageBackground() {
  "use strict";

  function hide(el) {
    el.classList.add("hide");
  }

  function backClickHandler(evt) {
    const back = evt.currentTarget;
    hide(back);
  }
  const cover = document.querySelector("");
  cover.addEventListener("click", backClickHandler);
}());

You need to provide more information here. What problems are you having with the code? What is it doing or not doing that you want it to do?

I think I figured out what I needed to do.
https://jsfiddle.net/Lq1b7cds/5/

I just needed to add this line in:
document.body.style.backgroundImage= "none";

(function manageBackground() {
  "use strict";

  function backClickHandler(evt) {
    document.body.style.backgroundColor = "green";
    document.body.style.backgroundImage= "none";

  }
  const back = document.querySelector(".jacketa");
  back.addEventListener("click", backClickHandler);
}());