html { 
  background: url(hintergrund.jpeg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

div {
   background: rgb(255, 255, 255);
   background: rgba(255, 255, 255, 0.5); 
   border-radius: 25px;
}