$width: 500px;
$height: 8px;
$transition: 3s linear;
$one: hsl(random(360), 75%, 50%);
$two: hsl(random(360), 75%, 50%);
body {
  background: #111;
}
hr {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  width: $width;
  height: $height;
  margin: auto;
  border-radius: $height;
  border: 0;
  background: linear-gradient(90deg, $one 0%, $two 50%, $one 100%);
  animation: scroll $transition infinite;
}
@keyframes scroll {
  100% {
    background-position: $width 0;
  }
}