body {
    margin: 0;
    display: flex;
    font-family: "Helvetica Neue", Arial, sans-serif;
    background-color: #f5f7fa;
    color: #333;
}

nav {
    width: 200px;
    height: 100vh;
    background-color: #2c3e50;
    color: white;
    position: fixed;
    left: 0;
    top: 0;
    padding: 20px;
    transition: 0.3s;
    z-index: 2000;
    overflow-y: auto;
    /* 目次が多い時用 */
}

nav h2 {
    font-size: 1.2rem;
    border-bottom: 1px solid #555;
    padding-bottom: 10px;
}

nav ul {
    list-style: none;
    padding: 0;
}

nav li {
    margin: 15px 0;
}

nav a {
    color: #ecf0f1;
    text-decoration: none;
    transition: 0.3s;
}

nav a:hover {
    color: #3498db;
}

.menu-toggle {
    display: none;
    position: fixed;
    top: 10px;
    left: 10px;
    background: #2c3e50;
    color: white;
    border: none;
    padding: 10px 15px;
    border-radius: 5px;
    cursor: pointer;
    z-index: 1000;
}

main {
    margin-left: 240px;
    /* nav(200px) + padding */
    padding: 40px;
    flex-grow: 1;
    transition: 0.3s;
    max-width: 1000px;
    /* 読みやすさのため */
}

.overlay {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100vw;
    height: 100vh;
    background: rgba(0, 0, 0, 0.5);
    z-index: 1500;
}

.overlay.active {
    display: block;
}

.subject-card {
    background: white;
    border-radius: 8px;
    padding: 20px;
    margin-bottom: 30px;
    box-shadow: 0 2px 5px rgba(0, 0, 0, 0.1);
}

.subject-card h2 {
    margin-top: 0;
    color: #2980b9;
    border-left: 5px solid #2980b9;
    padding-left: 15px;
}

.link-list {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(180px, 1fr));
    gap: 10px;
}

.link-list a {
    display: block;
    padding: 10px;
    background: #eee;
    text-decoration: none;
    color: #333;
    border-radius: 4px;
    text-align: center;
}

.link-list a:hover {
    background: #3498db;
    color: white;
    transform: translateY(-2px);
}

.text-background {
    display: inline-block;
    font-size: 2.5rem;
    font-weight: 600;
    line-height: 1.4em;
    padding: 5px;
    background: #fff8b6;
    font-size: larger;
}

table {
    width: 100%;
    border-collapse: collapse;
    font-size: 14px;
}

th,
td {
    border-bottom: 1px solid #eee;
    padding: 8px;
    text-align: left;
}

tr:nth-child(even) {
    background: #fafafa;
}

/* スマホ対応 */
@media (max-width: 768px) {
    .menu-toggle {
        display: block;
    }

    nav {
        left: -280px;
    }

    nav.open {
        left: 0;
    }

    main {
        margin-left: 0;
        padding: 60px 15px;
    }
}