koel/resources/assets/css/partials/skeleton.pcss

.skeleton {
  .pulse, &.pulse {
    animation: skeleton-pulse 2s infinite;
    background-color: rgba(255, 255, 255, .05);
  }

  @keyframes skeleton-pulse {
    0%, 100% {
      opacity: 0;
    }
    50% {
      opacity: 1;
    }
  }
}