.arena-background {
  position: absolute;
  inset: 0;
  z-index: -1;
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
  opacity: 0;
  transition: opacity .65s ease;
  pointer-events: none;
}
.arena-background.visible { opacity: 1; }
@media (prefers-reduced-motion: reduce) {
  .arena-background { transition: none; }
}