html, body {
  margin: 0;
  height: 100%;
  background: radial-gradient(circle at center, #020412 0%, #000 80%);
  overflow: hidden;
}

#universe {
  width: 100vw;
  height: 100vh;
  display: block;
  background: transparent;
}

#soundBtn {
  position: fixed;
  top: 1rem;
  left: 1rem;
  z-index: 10;
  padding: 10px 16px;
  font-size: 1rem;
  color: #fff;
  background: rgba(30, 30, 40, 0.6);
  border: 1px solid #444;
  border-radius: 20px;
  cursor: pointer;
  backdrop-filter: blur(6px);
  transition: all 0.2s ease;
}
#soundBtn:hover { background: rgba(60, 60, 80, 0.7); }
#soundBtn.on { background: rgba(0,150,100,0.7); border-color:#0f0; }