body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
    background-color: #f4f7f6;
    color: #333;
    margin: 0;
    line-height: 1.6;
}

header {
    background-color: #ffffff;
    padding: 1.5rem 2rem;
    text-align: center;
    border-bottom: 1px solid #e0e0e0;
}

header h1 {
    margin: 0;
    font-size: 2rem;
    color: #2c3e50;
}

#concepts-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 1.5rem;
    padding: 2rem;
    max-width: 1200px;
    margin: 0 auto;
}

.concept-card {
    background-color: #ffffff;
    border-radius: 8px;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
    padding: 1.5rem;
    cursor: pointer;
    transition: transform 0.2s ease-in-out, box-shadow 0.2s ease-in-out;
}

.concept-card:hover {
    transform: translateY(-5px);
    box-shadow: 0 8px 20px rgba(0, 0, 0, 0.12);
}

.concept-card h2 {
    margin: 0 0 0.5rem 0;
    font-size: 1.25rem;
    color: #34495e;
}

.modal {
    display: none;
    position: fixed;
    z-index: 1000;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    overflow: auto;
    background-color: rgba(0, 0, 0, 0.5);
    align-items: center;
    justify-content: center;
}

.modal.show {
    display: flex;
}

.modal-content {
    background-color: #ffffff;
    margin: auto;
    padding: 2rem 2.5rem;
    border-radius: 8px;
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.2);
    width: 90%;
    max-width: 600px;
    animation: slide-down 0.3s ease-out;
    position: relative;
}

.close-button {
    color: #aaa;
    position: absolute;
    top: 1rem;
    right: 1.5rem;
    font-size: 1.75rem;
    font-weight: bold;
    cursor: pointer;
}

.close-button:hover,
.close-button:focus {
    color: #333;
}

#modal-title {
    color: #2c3e50;
    border-bottom: 2px solid #3498db;
    padding-bottom: 0.5rem;
    margin-bottom: 1rem;
}

@keyframes slide-down {
    from {
        transform: translateY(-50px);
        opacity: 0;
    }
    to {
        transform: translateY(0);
        opacity: 1;
    }
}