html {
  background: url(../img/Background.jpg) no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

table {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%); /* Verschiebt das Element um seine eigene halbe Größe zurück */
}