mastodon-docker-playground/app/javascript/styles/mastodon/modal.scss

.modal-layout {
  background: $ui-base-color url('~images/wave-modal.png') repeat-x bottom fixed;
  display: flex;
  flex-direction: column;
  height: 100vh;
  padding: 0;
}

.modal-layout__mastodon {
  display: flex;
  flex: 1;
  flex-direction: column;
  justify-content: flex-end;

  > * {
    flex: 1;
    max-height: 235px;
    background: url('~images/mastodon-ui.png') no-repeat left bottom / contain;
  }
}