html { 
  background: url(bg.jpg) no-repeat center center fixed; 
  
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: 70%;

}

@media (max-width: 768px) {
  html {
    background-size: 50%;
  }
}