:root {
    --bg: #fafaf8;
    --card-bg: #ffffff;
    --text: #2c2c2c;
    --text-secondary: #555;
    --accent: #1a73e8;
    --accent-light: #e8f0fe;
    --border: #e0e0e0;
    --cheat-bg: #fff8e1;
    --cheat-border: #f9a825;
    --theory-bg: #e8f5e9;
    --theory-border: #43a047;
    --practice-bg: #e3f2fd;
    --practice-border: #1976d2;
    --answer-bg: #fce4ec;
    --answer-border: #e91e63;
    --code-bg: #f5f5f5;
    --shadow: 0 2px 8px rgba(0,0,0,0.08);
    --radius: 8px;
}

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

body {
    font-family: 'Segoe UI', 'Helvetica Neue', Arial, sans-serif;
    background: var(--bg);
    color: var(--text);
    line-height: 1.7;
    padding: 0;
    max-width: 960px;
    margin: 0 auto;
}

.ch-nav {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 16px 24px;
    background: var(--card-bg);
    border-bottom: 1px solid var(--border);
    position: sticky;
    top: 0;
    z-index: 100;
    box-shadow: var(--shadow);
}

.nav-link {
    text-decoration: none;
    color: var(--accent);
    font-weight: 600;
    padding: 8px 16px;
    border-radius: var(--radius);
    transition: background 0.2s;
}

.nav-link:hover {
    background: var(--accent-light);
}

.nav-link.disabled {
    visibility: hidden;
}

.nav-home {
    font-size: 0.9em;
}

.page-title {
    text-align: center;
    padding: 40px 24px 20px;
    font-size: 2em;
    font-weight: 700;
    color: var(--text);
}

.section {
    margin: 24px;
    padding: 32px;
    background: var(--card-bg);
    border-radius: var(--radius);
    box-shadow: var(--shadow);
    border-left: 4px solid var(--border);
}

.section-cheat {
    border-left-color: var(--cheat-border);
    background: linear-gradient(135deg, var(--cheat-bg) 0%, var(--card-bg) 100%);
}

.section-theory {
    border-left-color: var(--theory-border);
    background: linear-gradient(135deg, var(--theory-bg) 0%, var(--card-bg) 100%);
}

.section-practice {
    border-left-color: var(--practice-border);
    background: linear-gradient(135deg, var(--practice-bg) 0%, var(--card-bg) 100%);
}

.section-label {
    display: inline-block;
    padding: 4px 14px;
    border-radius: 20px;
    font-size: 0.8em;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    margin-bottom: 16px;
}

.label-cheat { background: var(--cheat-border); color: #fff; }
.label-theory { background: var(--theory-border); color: #fff; }
.label-practice { background: var(--practice-border); color: #fff; }

h1 { font-size: 1.7em; margin: 24px 0 16px; color: var(--text); border-bottom: 2px solid var(--border); padding-bottom: 8px; }
h2 { font-size: 1.35em; margin: 20px 0 12px; color: var(--text); }
h3 { font-size: 1.15em; margin: 16px 0 10px; color: var(--text-secondary); }
h4 { font-size: 1.05em; margin: 12px 0 8px; }

p { margin: 8px 0; }

table {
    width: 100%;
    border-collapse: collapse;
    margin: 16px 0;
    font-size: 0.95em;
}

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

th {
    background: var(--accent-light);
    font-weight: 600;
}

tr:nth-child(even) {
    background: #f9f9f9;
}

code {
    background: var(--code-bg);
    padding: 2px 6px;
    border-radius: 4px;
    font-size: 0.9em;
    font-family: 'Fira Code', 'SF Mono', 'Cascadia Code', monospace;
}

pre {
    background: var(--code-bg);
    padding: 16px 20px;
    border-radius: var(--radius);
    overflow-x: auto;
    margin: 12px 0;
    border: 1px solid var(--border);
}

pre code {
    background: none;
    padding: 0;
    font-size: 0.9em;
}

.math-block {
    margin: 16px 0;
    padding: 16px 24px;
    background: #fafafa;
    border-radius: var(--radius);
    border: 1px solid #e8e8e8;
    overflow-x: auto;
}

.math-block .math-line {
    margin: 6px 0;
    font-size: 1.1em;
}

.math-block .ref-line {
    margin: 4px 0;
    font-size: 0.95em;
    color: var(--text-secondary);
    padding-left: 8px;
    border-left: 2px solid var(--border);
}

hr {
    border: none;
    border-top: 1px solid var(--border);
    margin: 24px 0;
}

strong { color: #1a1a1a; }

ul, ol {
    margin: 8px 0;
    padding-left: 24px;
}

li {
    margin: 4px 0;
}

/* Answers collapsible panel */
details.answers-panel {
    margin-top: 32px;
    border: 2px solid var(--answer-border);
    border-radius: var(--radius);
    overflow: hidden;
}

details.answers-panel summary {
    background: var(--answer-border);
    color: #fff;
    padding: 14px 24px;
    font-weight: 700;
    font-size: 1.1em;
    cursor: pointer;
    user-select: none;
    list-style: none;
    display: flex;
    align-items: center;
    gap: 8px;
}

details.answers-panel summary::-webkit-details-marker {
    display: none;
}

details.answers-panel summary::before {
    content: "\25B6";
    font-size: 0.8em;
    transition: transform 0.2s;
}

details.answers-panel[open] summary::before {
    transform: rotate(90deg);
}

details.answers-panel .answers-content {
    padding: 24px;
    background: linear-gradient(135deg, #fff5f7 0%, #fff 100%);
}

details.answers-panel .answers-content br {
    display: block;
    margin-bottom: 4px;
}

/* Detailed solutions collapsible panel */
details.detailed-solutions {
    margin-top: 32px;
    border: 2px solid #5b7db1;
    border-radius: var(--radius);
    overflow: hidden;
}

details.detailed-solutions summary {
    background: #5b7db1;
    color: #fff;
    padding: 14px 24px;
    font-weight: 700;
    font-size: 1.1em;
    cursor: pointer;
    user-select: none;
    list-style: none;
    display: flex;
    align-items: center;
    gap: 8px;
}

details.detailed-solutions summary::-webkit-details-marker {
    display: none;
}

details.detailed-solutions summary::before {
    content: "\25B6";
    font-size: 0.8em;
    transition: transform 0.2s;
}

details.detailed-solutions[open] summary::before {
    transform: rotate(90deg);
}

details.detailed-solutions > :not(summary) {
    padding: 0 24px;
    background: linear-gradient(135deg, #f0f4fa 0%, #fff 100%);
}

details.detailed-solutions h3 {
    padding-top: 18px;
}

details.detailed-solutions hr {
    border: none;
    border-top: 1px solid #d0d8e8;
    margin: 20px 0;
}

details.russian-note {
    margin: 16px 0;
    border: 1px solid #81c784;
    border-radius: var(--radius);
    overflow: hidden;
}

details.russian-note summary {
    background: linear-gradient(135deg, #388e3c 0%, #66bb6a 100%);
    color: #fff;
    padding: 10px 18px;
    font-weight: 600;
    font-size: 0.95em;
    cursor: pointer;
    user-select: none;
    list-style: none;
    display: flex;
    align-items: center;
    gap: 8px;
}

details.russian-note summary::-webkit-details-marker {
    display: none;
}

details.russian-note summary::before {
    content: "\25B6";
    font-size: 0.75em;
    transition: transform 0.2s;
}

details.russian-note[open] summary::before {
    transform: rotate(90deg);
}

details.russian-note > :not(summary) {
    padding: 4px 18px;
    background: linear-gradient(135deg, #e8f5e9 0%, #fff 100%);
    font-size: 0.93em;
    line-height: 1.6;
}

/* Index page */
.index-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 20px;
    padding: 24px;
}

.ch-card {
    background: var(--card-bg);
    border-radius: var(--radius);
    box-shadow: var(--shadow);
    padding: 24px;
    text-decoration: none;
    color: var(--text);
    transition: transform 0.2s, box-shadow 0.2s;
    border-left: 4px solid var(--accent);
}

.ch-card:hover {
    transform: translateY(-4px);
    box-shadow: 0 6px 20px rgba(0,0,0,0.12);
}

.ch-card h2 {
    margin: 0 0 8px;
    color: var(--accent);
    border: none;
}

.ch-card p {
    color: var(--text-secondary);
    font-size: 0.9em;
    margin: 0;
}

.ch-card .badges {
    margin-top: 12px;
    display: flex;
    gap: 6px;
    flex-wrap: wrap;
}

.badge {
    font-size: 0.75em;
    padding: 2px 10px;
    border-radius: 12px;
    font-weight: 600;
}

.badge-cheat { background: var(--cheat-bg); color: #e65100; border: 1px solid var(--cheat-border); }
.badge-theory { background: var(--theory-bg); color: #2e7d32; border: 1px solid var(--theory-border); }
.badge-practice { background: var(--practice-bg); color: #1565c0; border: 1px solid var(--practice-border); }

/* Answer quicklink button */
.answer-hint {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 22px;
    height: 22px;
    border-radius: 50%;
    border: 1.5px solid var(--answer-border);
    background: transparent;
    color: var(--answer-border);
    font-size: 0.7em;
    font-weight: 700;
    cursor: pointer;
    margin-right: 6px;
    padding: 0;
    line-height: 1;
    vertical-align: middle;
    transition: all 0.15s;
    flex-shrink: 0;
}

.answer-hint:hover,
.answer-hint.active {
    background: var(--answer-border);
    color: #fff;
}

/* Theory quicklink button */
.theory-hint {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 22px;
    height: 22px;
    border-radius: 50%;
    border: 1.5px solid var(--theory-border);
    background: transparent;
    color: var(--theory-border);
    font-size: 0.7em;
    font-weight: 700;
    cursor: pointer;
    margin-right: 4px;
    padding: 0;
    line-height: 1;
    vertical-align: middle;
    transition: all 0.15s;
    flex-shrink: 0;
}

.theory-hint:hover,
.theory-hint.active {
    background: var(--theory-border);
    color: #fff;
}

/* Answer popup (inline below question) */
.answer-popup {
    display: none;
    margin: 4px 0 12px 28px;
    padding: 10px 16px;
    background: linear-gradient(135deg, #fff5f7 0%, #fff 100%);
    border: 1.5px solid var(--answer-border);
    border-radius: var(--radius);
    font-size: 0.95em;
    animation: popupFadeIn 0.15s ease;
}

.answer-popup br {
    display: block;
    margin-bottom: 3px;
}

/* Theory popup (inline below question) */
.theory-popup {
    display: none;
    margin: 4px 0 12px 28px;
    padding: 10px 16px;
    background: linear-gradient(135deg, var(--theory-bg) 0%, #fff 100%);
    border: 1.5px solid var(--theory-border);
    border-radius: var(--radius);
    font-size: 0.95em;
    animation: popupFadeIn 0.15s ease;
}

.theory-popup br {
    display: block;
    margin-bottom: 3px;
}

.theory-popup strong {
    color: var(--theory-border);
}

@keyframes popupFadeIn {
    from { opacity: 0; transform: translateY(-4px); }
    to { opacity: 1; transform: translateY(0); }
}

/* KaTeX overrides */
.katex { font-size: 1.1em !important; }
.katex-display { margin: 0.5em 0 !important; overflow-x: auto; overflow-y: hidden; }

/* Responsive */
@media (max-width: 640px) {
    .section { margin: 12px; padding: 20px; }
    .page-title { font-size: 1.5em; padding: 24px 16px 12px; }
    .ch-nav { padding: 12px 16px; }
    .index-grid { grid-template-columns: 1fr; padding: 12px; }
}

/* Print styles */
@media print {
    .ch-nav { display: none; }
    .section { box-shadow: none; border: 1px solid #ccc; break-inside: avoid; }
    details.answers-panel { open; }
    details.answers-panel[open] .answers-content { display: block; }
}

.build-version {
    text-align: center;
    color: var(--text-secondary);
    font-size: 0.75rem;
    padding: 16px 0 8px;
    opacity: 0.6;
}