.curtain {
    background: black;
    opacity: 50%;

    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;

    z-index: 1000;
}

.lds-ring {
  position: absolute;
  top: 50%;
  left: 50%;
  margin-left: -40px;
  margin-top: -40px;
}