#fullscreen {
  position: relative;
  border: 1px solid skyblue;
  padding: 1rem;
}

#fullscreen:-webkit-full-screen {
  background-color: yellow;
  display: grid;
  place-items: center;
}
#fullscreen:-moz-full-screen {
  background-color: yellow;
  display: grid;
  place-items: center;
}
#fullscreen:fullscreen {
  background-color: yellow;
  display: grid;
  place-items: center;
}
#fullscreen::backdrop {
  background-color:  skyblue;
}