body {
    font-family: system-ui, -apple-system, sans-serif;
    background-color: #f4f6f9;
    color: #333;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    margin: 0;
}

.tutorial-container {
    background: #ffffff;
    padding: 30px;
    border-radius: 12px;
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.08);
    width: 950px;
    max-width: 95%;
    text-align: center;
}

h2 { margin-top: 0; color: #1a202c; }
.subtitle { color: #718096; font-size: 0.95rem; margin-bottom: 20px; font-weight: 500; }

.visual-stage {
    display: flex;
    gap: 20px;
    justify-content: center;
    margin-bottom: 20px;
}

.panel {
    flex: 1;
    background: #f8fafc;
    border: 1px solid #e2e8f0;
    border-radius: 8px;
    padding: 15px;
    display: flex;
    flex-direction: column;
}

.panel h3 {
    font-size: 1rem;
    color: #4a5568;
    margin-top: 0;
    margin-bottom: 10px;
}

.panel img {
    width: 100%;
    height: auto;
    max-height: 240px;
    object-fit: contain;
    background: #ffffff;
    border: 1px solid #edf2f7;
    border-radius: 6px;
    padding: 5px;
}

.equation-container {
    margin-top: 12px;
    background: #ffffff;
    border: 1px solid #cbd5e0;
    border-radius: 6px;
    padding: 10px;
    font-size: 1.1rem;
    color: #1a202c;
    font-weight: bold;
    min-height: 40px;
    display: flex;
    align-items: center;
    justify-content: center;
}

.explanation-box {
    background: #ebf8ff;
    border-left: 4px solid #3182ce;
    padding: 15px;
    border-radius: 4px;
    text-align: left;
    margin-bottom: 20px;
    font-size: 1.05rem;
    color: #2b6cb0;
}

.controls {
    display: flex;
    justify-content: space-between;
}

button {
    background-color: #3182ce;
    color: white;
    border: none;
    padding: 10px 22px;
    font-size: 1rem;
    border-radius: 6px;
    cursor: pointer;
    transition: background 0.2s, opacity 0.2s;
}

button:hover { background-color: #2b6cb0; }
button:disabled { background-color: #cbd5e0; cursor: not-allowed; opacity: 0.7; }

@media (max-width: 768px) {
    .visual-stage {
        flex-direction: column;
    }
}