change the color of a square with onmouseover

Add an event parameter (e) to your mouseOver func and use e.target to get the box to set the background on

function getRandomColor() {
    let letters = "0123456789ABCDEF";
    let color = "#";
    for (let i = 0; i < 6; i++) {
        color += letters[Math.floor(Math.random() * 16)];
    }
    return color;
}

function mouseOver(e) {
  let newColor = getRandomColor();
  return e.target.style.backgroundColor = newColor;
}