.modal {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: 20;
  background: rgba(0,0,0,0.8);
  display: none;
}
.modal.visible {
  display: flex;
  justify-content: center;
  align-items: center;
}
.modal > div {
  background: #213;
  padding: 1rem;
}