#tree {
    width: 300px; margin: 40px auto; display: flex; flex-direction: column; gap: 8px;
}
.light {
    width: 80px; height: 80px; border-radius: 50%; margin: 0 auto;
    box-shadow: 0 0 20px currentColor;
}
.stage { background: #222; } /* pre-stage / stage bulbs */
.amber  { background: #ff0; }
.green  { background: #0f0; }
.red    { background: #f00; }