:root {
    --bg: #f4f5f7;
    --surface: #ffffff;
    --text: #1f2430;
    --muted: #6b7280;
    --border: #e3e6ec;
    --accent: #3b5bdb;
    --accent-soft: #edf1ff;
    --warning-bg: #fff7e6;
    --warning-border: #f0a500;
    --memory-bg: #e9f9f0;
    --memory-border: #16a34a;
    --shadow: 0 10px 30px rgba(20, 25, 45, 0.08);
}

[data-theme="dark"] {
    --bg: #14161c;
    --surface: #1c1f27;
    --text: #e6e8ee;
    --muted: #99a0b0;
    --border: #2c313c;
    --accent: #7d9bff;
    --accent-soft: #232a3d;
    --warning-bg: #2e2413;
    --warning-border: #d99400;
    --memory-bg: #14291d;
    --memory-border: #34c07a;
    --shadow: 0 10px 30px rgba(0, 0, 0, 0.35);
}

* {
    box-sizing: border-box;
}

body {
    margin: 0;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    background: var(--bg);
    color: var(--text);
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    line-height: 1.65;
    transition: background 0.2s ease, color 0.2s ease;
}

/* ---------- Top bar ---------- */

.topbar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 16px;
    padding: 14px 20px;
    background: var(--surface);
    border-bottom: 1px solid var(--border);
}

.brand {
    display: flex;
    align-items: baseline;
    gap: 10px;
}

.brand-mark {
    font-weight: 700;
    letter-spacing: 0.06em;
    color: var(--accent);
}

.brand-sub {
    font-size: 0.85rem;
    color: var(--muted);
}

.topbar-actions {
    display: flex;
    align-items: center;
    gap: 12px;
}

.search-trigger {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 7px 12px;
    border: 1px solid var(--border);
    border-radius: 8px;
    background: var(--surface);
    color: var(--muted);
    font-size: 0.85rem;
    font-family: inherit;
    cursor: pointer;
}

.search-trigger:hover {
    background: var(--accent-soft);
    color: var(--text);
}

.search-trigger kbd {
    padding: 1px 6px;
    border: 1px solid var(--border);
    border-radius: 5px;
    background: var(--bg);
    font-family: inherit;
    font-size: 0.75rem;
}

.icon-btn {
    width: 34px;
    height: 34px;
    border: 1px solid var(--border);
    border-radius: 8px;
    background: var(--surface);
    color: var(--text);
    font-size: 1rem;
    cursor: pointer;
}

.icon-btn:hover {
    background: var(--accent-soft);
}

/* ---------- Progress ---------- */

.progress {
    height: 3px;
    background: var(--border);
}

.progress-bar {
    height: 100%;
    width: 0;
    background: var(--accent);
    transition: width 0.25s ease;
}

/* ---------- Card ---------- */

.stage {
    flex: 1;
    display: flex;
    justify-content: center;
    padding: 28px 20px;
}

.card {
    width: 100%;
    max-width: 780px;
    padding: 36px 40px;
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 16px;
    box-shadow: var(--shadow);
}

.card > :first-child {
    margin-top: 0;
}

.card > :last-child {
    margin-bottom: 0;
}

.card h1 {
    font-size: 1.7rem;
    line-height: 1.3;
}

.card h2 {
    font-size: 1.2rem;
    margin-top: 1.8em;
}

.card hr {
    border: 0;
    border-top: 1px solid var(--border);
    margin: 1.6em 0;
}

.card blockquote {
    margin: 1.2em 0;
    padding: 12px 18px;
    border-left: 4px solid var(--accent);
    border-radius: 0 10px 10px 0;
    background: var(--accent-soft);
}

.card blockquote p {
    margin: 0;
}

.card table {
    width: 100%;
    border-collapse: collapse;
    margin: 1.2em 0;
    font-size: 0.95rem;
}

.card th,
.card td {
    padding: 10px 14px;
    border: 1px solid var(--border);
    text-align: left;
}

.card th {
    background: var(--accent-soft);
    font-weight: 600;
}

.card code {
    padding: 2px 6px;
    border-radius: 6px;
    background: var(--accent-soft);
    font-size: 0.9em;
}

.card pre {
    padding: 14px 16px;
    border-radius: 10px;
    background: var(--accent-soft);
    overflow-x: auto;
}

.card pre code {
    padding: 0;
    background: none;
}

.card img {
    max-width: 100%;
    border-radius: 10px;
}

.card .warning,
.card .memory {
    margin: 1.2em 0;
    padding: 14px 18px;
    border-radius: 12px;
    border-left: 4px solid;
}

.card .warning {
    background: var(--warning-bg);
    border-left-color: var(--warning-border);
}

.card .memory {
    background: var(--memory-bg);
    border-left-color: var(--memory-border);
}

.card .warning > :first-child,
.card .memory > :first-child {
    margin-top: 0;
}

.card .warning > :last-child,
.card .memory > :last-child {
    margin-bottom: 0;
}

.status {
    color: var(--muted);
    text-align: center;
    margin: 0;
}

.status-error {
    color: #d1495b;
}

.status-error code {
    background: var(--accent-soft);
    padding: 2px 6px;
    border-radius: 6px;
}

/* ---------- Pager ---------- */

.pager {
    position: sticky;
    bottom: 0;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 18px;
    padding: 14px 20px;
    background: var(--surface);
    border-top: 1px solid var(--border);
}

.nav-btn {
    min-width: 130px;
    padding: 10px 18px;
    border: 1px solid var(--border);
    border-radius: 10px;
    background: var(--surface);
    color: var(--text);
    font-size: 0.95rem;
    font-weight: 500;
    cursor: pointer;
    transition: background 0.15s ease, transform 0.1s ease;
}

.nav-btn:hover:not(:disabled) {
    background: var(--accent-soft);
}

.nav-btn:active:not(:disabled) {
    transform: translateY(1px);
}

.nav-btn:disabled {
    opacity: 0.45;
    cursor: not-allowed;
}

.nav-btn-primary {
    background: var(--accent);
    border-color: var(--accent);
    color: #fff;
}

.nav-btn-primary:hover:not(:disabled) {
    background: var(--accent);
    filter: brightness(1.08);
}

.counter {
    min-width: 76px;
    text-align: center;
    font-variant-numeric: tabular-nums;
    color: var(--muted);
    font-size: 0.9rem;
}

/* ---------- Card metadata ---------- */

.meta {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    gap: 8px;
    margin-bottom: 24px;
    padding-bottom: 18px;
    border-bottom: 1px solid var(--border);
}

.chip {
    padding: 4px 11px;
    border-radius: 999px;
    background: var(--accent-soft);
    color: var(--text);
    font-size: 0.76rem;
    font-weight: 600;
    letter-spacing: 0.01em;
}

.chip-subject {
    background: var(--accent);
    color: #fff;
}

.chip-easy {
    background: var(--memory-bg);
    color: var(--memory-border);
}

.chip-medium {
    background: var(--warning-bg);
    color: var(--warning-border);
}

.chip-hard {
    background: #fdecef;
    color: #d1495b;
}

[data-theme="dark"] .chip-hard {
    background: #33191d;
    color: #f28b9c;
}

.chip-yield-high {
    background: transparent;
    border: 1px solid var(--accent);
    color: var(--accent);
}

.chip-yield-medium,
.chip-yield-low {
    background: transparent;
    border: 1px solid var(--border);
    color: var(--muted);
}

.tag {
    color: var(--muted);
    font-size: 0.76rem;
}

/* ---------- Quiz ---------- */

.quiz {
    margin-top: 36px;
    padding-top: 28px;
    border-top: 2px solid var(--border);
}

.quiz-head {
    display: flex;
    align-items: baseline;
    justify-content: space-between;
    gap: 12px;
    flex-wrap: wrap;
    margin-bottom: 20px;
}

.quiz-title {
    margin: 0;
    font-size: 1.2rem;
}

.quiz-score {
    color: var(--muted);
    font-size: 0.82rem;
    font-variant-numeric: tabular-nums;
}

.quiz-q {
    margin-bottom: 24px;
    padding: 18px 20px;
    border: 1px solid var(--border);
    border-radius: 12px;
    background: var(--bg);
}

.quiz-prompt {
    margin: 0 0 14px;
    font-weight: 500;
}

.quiz-label {
    display: inline-block;
    min-width: 34px;
    color: var(--accent);
    font-weight: 700;
    font-size: 0.85rem;
}

.quiz-options {
    display: flex;
    flex-direction: column;
    gap: 8px;
}

.quiz-option {
    padding: 10px 14px;
    border: 1px solid var(--border);
    border-radius: 9px;
    background: var(--surface);
    color: var(--text);
    font-family: inherit;
    font-size: 0.92rem;
    text-align: left;
    cursor: pointer;
    transition: background 0.15s ease, border-color 0.15s ease;
}

.quiz-option:hover:not(:disabled) {
    border-color: var(--accent);
    background: var(--accent-soft);
}

.quiz-option:disabled {
    cursor: default;
}

.quiz-option.is-correct {
    border-color: var(--memory-border);
    background: var(--memory-bg);
    font-weight: 600;
}

.quiz-option.is-correct::after {
    content: " ✓";
    color: var(--memory-border);
}

.quiz-option.is-wrong {
    border-color: #d1495b;
    background: var(--warning-bg);
}

.quiz-option.is-wrong::after {
    content: " ✕";
    color: #d1495b;
}

.quiz-explanation {
    margin-top: 14px;
    padding: 12px 16px;
    border-left: 3px solid var(--accent);
    border-radius: 0 9px 9px 0;
    background: var(--accent-soft);
    font-size: 0.9rem;
}

.quiz-explanation > :first-child {
    margin-top: 0;
}

.quiz-explanation > :last-child {
    margin-bottom: 0;
}

/* ---------- Search overlay ---------- */

.overlay {
    position: fixed;
    inset: 0;
    z-index: 50;
    display: flex;
    justify-content: center;
    align-items: flex-start;
    padding: 12vh 20px 20px;
    background: rgba(15, 18, 28, 0.45);
    backdrop-filter: blur(2px);
}

.overlay[hidden] {
    display: none;
}

.search-panel {
    width: 100%;
    max-width: 560px;
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 14px;
    box-shadow: var(--shadow);
    overflow: hidden;
}

.search-input {
    width: 100%;
    padding: 16px 20px;
    border: 0;
    border-bottom: 1px solid var(--border);
    background: transparent;
    color: var(--text);
    font-family: inherit;
    font-size: 1rem;
}

.search-input:focus {
    outline: none;
}

.search-results {
    max-height: 46vh;
    margin: 0;
    padding: 6px;
    list-style: none;
    overflow-y: auto;
}

.search-result {
    display: flex;
    align-items: baseline;
    gap: 12px;
    padding: 9px 14px;
    border-radius: 8px;
    cursor: pointer;
    font-size: 0.92rem;
}

.search-result.is-active,
.search-result:hover {
    background: var(--accent-soft);
}

.search-num {
    min-width: 42px;
    color: var(--muted);
    font-variant-numeric: tabular-nums;
    font-size: 0.8rem;
}

.search-title {
    flex: 1;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}

.search-subject {
    padding: 2px 9px;
    border-radius: 999px;
    background: var(--accent-soft);
    color: var(--muted);
    font-size: 0.72rem;
    white-space: nowrap;
}

.search-empty {
    padding: 16px;
    text-align: center;
    color: var(--muted);
    font-size: 0.9rem;
}

.search-hint {
    margin: 0;
    padding: 10px 16px;
    border-top: 1px solid var(--border);
    color: var(--muted);
    font-size: 0.75rem;
    text-align: center;
}

@media (max-width: 640px) {
    .card {
        padding: 24px 20px;
    }

    .search-trigger kbd {
        display: none;
    }

    .nav-btn {
        min-width: 0;
        flex: 1;
    }
}