koel/resources/assets/sass/partials/_skeleton.scss

.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;
    }
  }
}