/* ===== Biến & reset ===== */
:root {
    --color-paper: #fffef9;
    --color-paper-edge: #e8e4d9;
    --color-ink: #1a1a1a;
    --color-ink-muted: #4a4a4a;
    --color-chapter: #0d47a1;
    --color-accent: #1565c0;
    --color-link-hover: #b71c1c;
    --font-serif: "Times New Roman", Times, Georgia, serif;
    --font-sans: Arial, Helvetica, sans-serif;
    --page-max-width: 720px;
    --toc-indent: 1.75rem;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: var(--font-serif);
    font-size: 17px;
    line-height: 1.5;
    color: var(--color-ink);
    background: #d4cfc4;
    min-height: 100vh;
    padding: 2rem 1rem 3rem;
}

/* ===== Trang bìa / tiêu đề sách ===== */
.book-header {
    max-width: var(--page-max-width);
    margin: 0 auto 1.5rem;
    text-align: center;
    padding: 2rem 1.5rem 1.25rem;
    background: var(--color-paper);
    border: 1px solid var(--color-paper-edge);
    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.12);
}

.book-header__grade {
    font-family: var(--font-sans);
    font-size: 0.85rem;
    letter-spacing: 0.15em;
    text-transform: uppercase;
    color: var(--color-ink-muted);
    margin-bottom: 0.5rem;
}

.book-header__title {
    font-size: 1.65rem;
    font-weight: bold;
    text-transform: uppercase;
    letter-spacing: 0.04em;
    color: var(--color-chapter);
    margin-bottom: 0.35rem;
}

.book-header__series {
    font-size: 1rem;
    font-style: italic;
    color: var(--color-ink-muted);
}

/* ===== Trang mục lục ===== */
.toc-page {
    max-width: var(--page-max-width);
    margin: 0 auto;
    padding: 2.25rem 2rem 2.5rem;
    background: var(--color-paper);
    border: 1px solid var(--color-paper-edge);
    box-shadow: 0 2px 12px rgba(0, 0, 0, 0.14);
}

.toc-page__heading {
    font-size: 1.75rem;
    font-weight: bold;
    text-align: center;
    letter-spacing: 0.35em;
    text-indent: 0.35em;
    margin-bottom: 2rem;
    padding-bottom: 0.75rem;
    border-bottom: 2px solid var(--color-ink);
}

/* ===== Chủ đề ===== */
.toc-chapter {
    margin-bottom: 1.35rem;
}

.toc-chapter:last-child {
    margin-bottom: 0;
}

.toc-chapter__title {
    font-size: 1.05rem;
    font-weight: bold;
    color: var(--color-chapter);
    margin-bottom: 0.45rem;
    padding-left: 0;
    line-height: 1.4;
}

/* ===== Danh sách bài ===== */
.toc-lessons {
    list-style: none;
    padding-left: var(--toc-indent);
}

.toc-item {
    margin-bottom: 0.2rem;
}

.toc-link {
    display: flex;
    align-items: baseline;
    gap: 0.25rem;
    text-decoration: none;
    color: var(--color-ink);
    padding: 0.2rem 0;
    transition: color 0.2s;
}

.toc-link:hover,
.toc-link:focus {
    color: var(--color-link-hover);
}

.toc-link:hover .toc-dots,
.toc-link:focus .toc-dots {
    border-bottom-color: var(--color-link-hover);
}

.toc-text {
    flex-shrink: 0;
}

/* Số trang bên phải (điền trong .toc-page-num) */
.toc-page-ref {
    flex-shrink: 0;
    white-space: nowrap;
    font-size: 0.95rem;
    color: var(--color-ink-muted);
}

.toc-page-num {
    display: inline-block;
    min-width: 1.25rem;
    text-align: right;
    font-weight: normal;
    color: var(--color-ink);
}

.toc-link:hover .toc-page-ref,
.toc-link:focus .toc-page-ref {
    color: var(--color-link-hover);
}

.toc-link:hover .toc-page-num,
.toc-link:focus .toc-page-num {
    color: var(--color-link-hover);
}

/* Chấm nối kiểu mục lục SGK */
.toc-dots {
    flex: 1 1 auto;
    min-width: 1.5rem;
    margin: 0 0.35rem;
    border-bottom: 1px dotted var(--color-ink-muted);
    transform: translateY(-0.2em);
    opacity: 0.75;
}

/* Bài đặc biệt (bảng thuật ngữ, v.v.) */
.toc-item--special .toc-text {
    font-style: italic;
}

/* ===== Chân trang ===== */
.book-footer {
    max-width: var(--page-max-width);
    margin: 1.25rem auto 0;
    text-align: center;
    font-family: var(--font-sans);
    font-size: 0.8rem;
    color: var(--color-ink-muted);
}

/* ===== Responsive ===== */
@media (max-width: 600px) {
    body {
        padding: 1rem 0.5rem 2rem;
        font-size: 16px;
    }

    .book-header {
        padding: 1.5rem 1rem;
    }

    .book-header__title {
        font-size: 1.35rem;
    }

    .toc-page {
        padding: 1.5rem 1.15rem 1.75rem;
    }

    .toc-page__heading {
        font-size: 1.45rem;
        letter-spacing: 0.2em;
        text-indent: 0.2em;
        margin-bottom: 1.5rem;
    }

    .toc-lessons {
        padding-left: 1rem;
    }

    .toc-dots {
        min-width: 0.75rem;
    }
}