body {
  background-image: url("background420.png");
  color: #8b0000;
}
mark {
  background-color: darkred;
  color: white;
}
div {
    height: 200px;
    width: 400px;
    background: black;

    position: fixed;
    top: 50%;
    left: 50%;
    margin-top: -100px;
    margin-left: -200px;
}