Part 3 - Classes that will animate the underlay and close icon

Post.scss

.page-post {
  &.post-exit,
  &.post-enter {
    .content,
    .close,
    .underlay {
      opacity: 0;
    }
    .underlay {
      transition-delay: 0;
    }
  }
  &.post-enter-started {
    .content,
    .underlay {
      opacity: 1;
      transition-delay: 100ms;
    }
    .close {
      opacity: 0.7;
    }
  }
}