html, body {
  height: 100%;
  margin: 0;
  background: #000;
  overflow: hidden;
  font-family: system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial, sans-serif;
}

#stage {
  position: fixed;
  inset: 0;
  background: #000;
}

.bg-layer {
  position: absolute;
  inset: 0;
  background-position: center center;
  background-repeat: no-repeat;
  background-size: cover;
  opacity: 0;
  transition: opacity 2.5s ease-in-out;
  will-change: opacity;
}

.overlay {
  position: fixed;
  inset: 0;
  display: grid;
  place-items: center;
  z-index: 50;
  background: radial-gradient(
    circle at center,
    rgba(0,0,0,0.12) 0%,
    rgba(0,0,0,0.35) 55%,
    rgba(0,0,0,0.60) 100%
  );
}

.overlay.hidden {
  display: none;
}

.start-btn {
  appearance: none;
  border: 1px solid rgba(255,255,255,0.22);
  color: rgba(255,255,255,0.92);
  background: rgba(0,0,0,0.18);
  padding: 18px 34px;
  border-radius: 999px;
  font-size: 18px;
  cursor: pointer;
  user-select: none;

  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);

  box-shadow:
    0 10px 30px rgba(0,0,0,0.35),
    0 0 0 1px rgba(255,255,255,0.06) inset;

  transition: transform 180ms ease, background 180ms ease, border-color 180ms ease;
}

.start-btn:hover {
  transform: translateY(-1px);
  background: rgba(0,0,0,0.26);
  border-color: rgba(255,255,255,0.32);
}

.start-btn:active { transform: translateY(0px); }

.start-btn:focus-visible {
  outline: none;
  border-color: rgba(255,255,255,0.50);
  box-shadow:
    0 12px 34px rgba(0,0,0,0.38),
    0 0 0 3px rgba(255,255,255,0.12);
}

/* Fullscreen button (YouTube-ish: bottom-right, small, unobtrusive) */
.fs-btn {
  position: fixed;
  right: 18px;
  bottom: 18px;
  z-index: 60;

  width: 42px;
  height: 42px;
  border-radius: 12px;

  border: 1px solid rgba(255,255,255,0.18);
  background: rgba(0,0,0,0.14);
  color: rgba(255,255,255,0.85);

  display: grid;
  place-items: center;

  cursor: pointer;
  user-select: none;

  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);

  opacity: 0.70;
  transition: opacity 180ms ease, transform 180ms ease, border-color 180ms ease, background 180ms ease;
}

.fs-btn:hover {
  opacity: 0.95;
  transform: translateY(-1px);
  background: rgba(0,0,0,0.22);
  border-color: rgba(255,255,255,0.28);
}

.fs-btn:active { transform: translateY(0px); }

.fs-btn:focus-visible {
  outline: none;
  opacity: 1;
  border-color: rgba(255,255,255,0.45);
  box-shadow: 0 0 0 3px rgba(255,255,255,0.12);
}

.fs-icon {
  width: 20px;
  height: 20px;
  fill: currentColor;
  display: none;
}

/* Show correct icon based on fullscreen state */
body.is-fullscreen .fs-exit { display: block; }
body:not(.is-fullscreen) .fs-enter { display: block; }