app.css

.App:after {
  content: '';
  transition: opacity 450ms cubic-bezier(0.23, 1, 0.32, 1),
              transform 0ms cubic-bezier(0.23, 1, 0.32, 1) 450ms;
  position: fixed; top: 0; right: 0; bottom: 0; left: 0;
  background-color: rgba(0, 0, 0, 0.33);
  transform: translateX(-100%);
  opacity: 0;
  z-index: 1;
}

.App.overlay:after {
  transition: opacity 450ms cubic-bezier(0.23, 1, 0.32, 1);
  transform: translateX(0%);
  opacity: 1;
}