body {
    margin: 0;
    padding: 0;
    min-height: 100vh;
    background-color: black;
    display: flex;
    justify-content: center;
    align-items: center;
    overflow: hidden;
}

/* ── Back Button ── */
.back-btn {
    position: fixed;
    top: 1rem;
    left: 1rem;
    z-index: 100;
    display: flex;
    align-items: center;
    justify-content: center;
    width: 32px;
    height: 32px;
    border: 1px solid rgba(255, 255, 255, 0.15);
    border-radius: 6px;
    background: rgba(255, 255, 255, 0.05);
    opacity: 0.3;
    transition: opacity 0.3s, border-color 0.3s;
}

.back-btn:hover {
    opacity: 1;
    border-color: rgba(255, 255, 255, 0.4);
}

.back-btn img {
    width: 14px;
    height: 14px;
    filter: brightness(0) invert(0.7);
}

/* ── Cube ── */
.container {
    perspective: 1000px;
}

.spinner {
    width: 400px;
    height: 400px;
    position: relative;
    transform-style: preserve-3d;
    animation: spin 12s infinite linear;
}

.spinner > div {
    background-color: rgba(255, 255, 255, 0.1);
    height: 100%;
    width: 100%;
    position: absolute;
    border: 2px solid white;
}

.spinner > div:nth-child(1) {
    transform: translateZ(-200px) rotateY(180deg);
}

.spinner > div:nth-child(2) {
    transform: rotateY(-90deg) translateZ(200px);
}

.spinner > div:nth-child(3) {
    transform: rotateY(90deg) translateZ(200px);
}

.spinner > div:nth-child(4) {
    transform: rotateX(90deg) translateZ(200px);
}

.spinner > div:nth-child(5) {
    transform: rotateX(-90deg) translateZ(200px);
}

.spinner > div:nth-child(6) {
    transform: translateZ(200px);
}

@keyframes spin {
    from { transform: rotateX(0deg) rotateY(0deg) rotateZ(0deg); }
    to   { transform: rotateX(360deg) rotateY(720deg) rotateZ(1080deg); }
}