:root {
  --bg-color: black;
  --transition-duration: 1000ms;
}

html, body {
  margin: 0;
  padding: 0;
  overflow: hidden;
  background-color: var(--bg-color);
}

#frame, #frame img {
  position: absolute;
  top:0; left: 0; right: 0; bottom: 0;
  margin: auto;
  width: 100%;
  height: 100%;
  object-fit: contain;
}

#frame img {
  opacity: 0;
  transition: opacity var(--transition-duration) ease-in-out;
}

#frame img.visible {
  opacity: 1;
}

#controls {
  position: absolute;
  bottom: 20px;
  left: 50%;
  transform: translateX(-50%);
  z-index: 10;
}

#controls button {
  font-size: 1.4rem;
  margin: 0 5px;
  padding: 10px;
  cursor: pointer;
  background: rgba(255,255,255,0.4);
  border: none;
  border-radius: 4px;
}