body{margin:0;display:flex;justify-content:center;align-items:center;height:100vh;background:#000;}
canvas{background:#fff;border:2px solid #000;}
/* Start screen */
#startScreen{
  position:absolute;top:50%;left:50%;transform:translate(-50%,-50%);
  width:800px;height:600px;
  background:url('start_bg.png') center/cover no-repeat;
  display:flex;flex-direction:column;align-items:center;justify-content:center;
}