:root {
  --star-color: #FFF;
  --animation-duration: 50s;
}

.bg-stars {
  position: absolute;
  opacity: .75;
  top: 0;
  height: 100%;
}

.stars {
  position: absolute;
  width: 1px;
  height: 1px;
  background: transparent;
  animation: animStar var(--animation-duration) linear infinite;
}

.stars:nth-child(2) {
  width: 2px;
  height: 2px;
  --animation-duration: 100s;
}

.stars:nth-child(3) {
  width: 3px;
  height: 3px;
  --animation-duration: 150s;
}

.stars:nth-child(4) {
  width: 1px;
  height: 1px;
  --animation-duration: 600s;
}

.stars:after {
  content: " ";
  position: absolute;
  top: 2000px;
  width: inherit;
  height: inherit;
  background: transparent;
}
@-webkit-keyframes animStar {
  from {
    -webkit-transform: translateY(0px);
  }
  to {
    -webkit-transform: translateY(-2000px);
  }
}
@-moz-keyframes animStar {
  from {
    -moz-transform: translateY(0px);
  }
  to {
    -moz-transform: translateY(-2000px);
  }
}
@-ms-keyframes animStar {
  from {
    -ms-transform: translateY(0px);
  }
  to {
    -ms-transform: translateY(-2000px);
  }
}
@keyframes animStar {
  from {
    transform: translateY(0px);
  }
  to {
    transform: translateY(-2000px);
  }
}