#canvas1 {
  width: 800px;
  height: 500px;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 800px;
  height: 500px;
  border: 2px solid black;
  background: linear-gradient(to bottom, lightblue, darkblue, black);
}