.pulse {
  background-color: yellow;
  animation-name: color;
  animation-duration: 2s;
  animation-iteration-count: infinite;
}

@keyframes color {
  0% {
    background-color: #3d3d3d;
  }
  50% {
    background-color: #224;
  }
  100% {
    background-color: #3d3d3d;
  }
}