.principal{
  background-image: url(./imagenes/dac.png);
  background-repeat: no-repeat;
  background-size: 100% 100%;
  background-attachment: fixed;
}
.principal2{
  background-image: url(./imagenes/dac.png) !important;
  background-repeat: no-repeat;
  background-size: 100% 100%;
  background-attachment: fixed;
}