bookstack/resources/assets/sass/_animations.scss

.anim.fadeIn {
  opacity: 0;
  animation-name: fadeIn;
  animation-duration: 160ms;
  animation-timing-function: ease-in-out;
  animation-fill-mode: forwards;
}

@keyframes fadeIn {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}