:root {
    --app-bg: #fdfbf7;
    --text-primary: #2d3436;
    --text-secondary: #636e72;
    --infinite-blue: #0984e3;
    --boundless-red: #d63031;
    --aleph-gold: #f39c12;
    --continuum-purple: #8e44ad;
    --shadow-sm: 0 4px 6px rgba(0, 0, 0, 0.05);
    --shadow-md: 0 10px 20px rgba(0, 0, 0, 0.08);
    --font-family: "Noto Serif SC", "Outfit", serif;
    --font-mono: "JetBrains Mono", monospace;
    --font-title: "Ma Shan Zheng", cursive;
}

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

body {
    font-family: var(--font-family);
    background: linear-gradient(135deg, #1e3c72 0%, #2a5298 100%);
    color: var(--text-primary);
    height: 100vh;
    overflow: hidden;
    display: flex;
    justify-content: center;
    align-items: center;
}

.app-container {
    width: 95vw;
    height: 90vh;
    max-width: 1600px;
    background: rgba(255, 252, 245, 0.98);
    backdrop-filter: blur(20px);
    border-radius: 24px;
    box-shadow: var(--shadow-md);
    display: flex;
    flex-direction: column;
    overflow: hidden;
}

.main-header {
    padding: 1.2rem 2rem;
    display: flex;
    justify-content: space-between;
    align-items: center;
    border-bottom: 1px solid rgba(0, 0, 0, 0.08);
    background: rgba(255, 255, 255, 0.6);
}

.header-content h1 {
    font-family: var(--font-title);
    font-size: 2rem;
    color: var(--infinite-blue);
}

.highlight {
    color: var(--boundless-red);
}

.subtitle {
    font-size: 0.9rem;
    color: var(--text-secondary);
    font-style: italic;
    margin-top: -5px;
}

.header-controls {
    display: flex;
    gap: 16px;
    align-items: center;
}

.mode-nav {
    display: flex;
    background: #fff;
    padding: 4px;
    border-radius: 12px;
    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.05);
    gap: 4px;
}

.nav-btn {
    display: flex;
    align-items: center;
    gap: 6px;
    padding: 8px 16px;
    border: none;
    background: transparent;
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.3s;
    font-family: var(--font-family);
    font-weight: 600;
    font-size: 0.85rem;
    color: var(--text-secondary);
}

.nav-btn.active {
    background: var(--infinite-blue);
    color: white;
    box-shadow: 0 4px 12px rgba(9, 132, 227, 0.3);
}

.nav-btn[data-mode="uncountable"].active {
    background: var(--boundless-red);
}

.nav-btn[data-mode="equipotence"].active {
    background: var(--aleph-gold);
}

.nav-btn[data-mode="cantor"].active {
    background: var(--continuum-purple);
}

.apple-btn {
    width: 40px;
    height: 40px;
    border-radius: 10px;
    border: none;
    background: #fff;
    cursor: pointer;
    font-size: 1.2rem;
    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.05);
    transition: all 0.2s;
}

.apple-btn:hover {
    transform: scale(1.1);
}

.main-stage {
    flex: 1;
    display: grid;
    grid-template-columns: 1fr 350px;
    gap: 2px;
    background: rgba(0, 0, 0, 0.02);
    overflow: hidden;
}

.demo-view {
    display: none;
    flex-direction: column;
    padding: 32px;
    background: rgba(255, 255, 255, 0.5);
    overflow-y: auto;
    gap: 24px;
}

.demo-view.active {
    display: flex;
}

.demo-title {
    text-align: center;
    margin-bottom: 16px;
}

.demo-title h2 {
    font-size: 1.5rem;
    color: var(--text-primary);
    margin-bottom: 8px;
}

.demo-title p {
    color: var(--text-secondary);
    font-size: 0.9rem;
}

/* Countable Sets */
.set-display-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 20px;
}

.set-box {
    background: #fff;
    padding: 20px;
    border-radius: 16px;
    box-shadow: var(--shadow-sm);
    border-top: 3px solid var(--infinite-blue);
}

.set-box h3 {
    font-size: 1.1rem;
    margin-bottom: 12px;
    color: var(--text-primary);
}

.elements-list {
    display: flex;
    gap: 12px;
    justify-content: center;
    margin: 16px 0;
    flex-wrap: wrap;
}

.elements-list span {
    background: rgba(9, 132, 227, 0.1);
    padding: 8px 12px;
    border-radius: 8px;
    font-family: var(--font-mono);
    font-weight: 600;
    color: var(--infinite-blue);
}

.set-note {
    text-align: center;
    font-size: 0.85rem;
    color: var(--text-secondary);
    margin-top: 12px;
    font-style: italic;
}

/* Bijection Builder */
.bijection-builder {
    background: #fff;
    padding: 24px;
    border-radius: 16px;
    box-shadow: var(--shadow-sm);
}

.bijection-builder h3 {
    text-align: center;
    margin-bottom: 20px;
    color: var(--text-primary);
}

.bijection-demo {
    margin: 20px 0;
}

.mapping-row {
    display: flex;
    align-items: center;
    gap: 20px;
}

.domain-items,
.codomain-items {
    flex: 1;
    display: flex;
    gap: 8px;
    flex-wrap: wrap;
    justify-content: center;
}

.domain-items span,
.codomain-items span {
    width: 44px;
    height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: rgba(9, 132, 227, 0.1);
    border: 2px solid var(--infinite-blue);
    border-radius: 8px;
    font-family: var(--font-mono);
    font-weight: 600;
    transition: all 0.3s;
}

.mapping-arrows {
    width: 100px;
    height: 200px;
    position: relative;
}

.demo-btn {
    width: 100%;
    padding: 12px;
    background: var(--infinite-blue);
    color: white;
    border: none;
    border-radius: 10px;
    font-family: var(--font-family);
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s;
}

.demo-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(9, 132, 227, 0.3);
}

/* Diagonal Proof */
.diagonal-proof {
    background: #fff;
    padding: 24px;
    border-radius: 16px;
    box-shadow: var(--shadow-sm);
}

.diagonal-proof h3 {
    text-align: center;
    margin-bottom: 20px;
}

.proof-stage {
    min-height: 300px;
    background: #f8f9fa;
    border-radius: 12px;
    padding: 20px;
    position: relative;
    overflow: auto;
}

.proof-table {
    font-family: var(--font-mono);
    font-size: 0.9rem;
}

.proof-controls {
    margin-top: 16px;
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.step-indicator {
    text-align: center;
    font-size: 0.85rem;
    color: var(--text-secondary);
    min-height: 20px;
}

.proof-conclusion {
    margin-top: 16px;
    padding: 16px;
    background: rgba(214, 48, 49, 0.1);
    border-left: 4px solid var(--boundless-red);
    border-radius: 8px;
    font-weight: 600;
    color: var(--boundless-red);
}

/* Cardinality Comparison */
.cardinality-comparison {
    background: #fff;
    padding: 24px;
    border-radius: 16px;
    box-shadow: var(--shadow-sm);
    display: flex;
    flex-direction: column;
    gap: 24px;
}

.comparison-row {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 16px;
}

.set-card {
    background: rgba(9, 132, 227, 0.05);
    padding: 20px;
    border-radius: 12px;
    border: 2px solid var(--infinite-blue);
    min-width: 150px;
    text-align: center;
}

.set-card.highlight {
    border-color: var(--boundless-red);
    background: rgba(214, 48, 49, 0.05);
}

.set-card h3 {
    font-size: 1rem;
    margin-bottom: 8px;
}

.cardinality-label {
    font-family: var(--font-mono);
    font-size: 1.1rem;
    font-weight: 700;
    color: var(--aleph-gold);
    margin-top: 8px;
}

.equal-sign,
.greater-sign {
    font-size: 2rem;
    font-weight: bold;
    color: var(--text-secondary);
}

.ethnic-analogy {
    background: linear-gradient(135deg, #fff5f5 0%, #fff 100%);
    padding: 20px;
    border-radius: 12px;
    border-left: 4px solid var(--boundless-red);
}

.ethnic-analogy h3 {
    color: var(--boundless-red);
    margin-bottom: 8px;
}

/* Power Set Demo */
.power-set-demo {
    background: #fff;
    padding: 24px;
    border-radius: 16px;
    box-shadow: var(--shadow-sm);
    display: flex;
    flex-direction: column;
    gap: 24px;
}

.theorem-box {
    background: linear-gradient(135deg, #f0f3ff 0%, #fff 100%);
    padding: 24px;
    border-radius: 12px;
    text-align: center;
    border: 2px solid var(--continuum-purple);
}

.theorem-statement {
    font-size: 1.5rem;
    font-family: var(--font-mono);
    color: var(--continuum-purple);
    margin-bottom: 12px;
}

.visual-demo {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 20px;
}

.base-set,
.power-set {
    background: #f8f9fa;
    padding: 20px;
    border-radius: 12px;
    text-align: center;
    width: 100%;
}

.set-elements,
.subset-grid {
    display: flex;
    gap: 8px;
    justify-content: center;
    flex-wrap: wrap;
    margin: 16px 0;
}

.set-elements span,
.subset-grid span {
    background: rgba(142, 68, 173, 0.1);
    padding: 8px 12px;
    border-radius: 8px;
    font-family: var(--font-mono);
    border: 1px solid var(--continuum-purple);
}

.arrow-down {
    font-size: 2rem;
    color: var(--continuum-purple);
}

.progress-analogy {
    padding: 20px;
    background: linear-gradient(135deg, #fff5f5 0%, #fff 100%);
    border-radius: 12px;
    border-left: 4px solid var(--boundless-red);
}

.progress-analogy h3 {
    color: var(--boundless-red);
    margin-bottom: 8px;
}

/* Insight Panel */
.insight-panel {
    background: rgba(255, 255, 255, 0.9);
    padding: 24px;
    display: flex;
    flex-direction: column;
    gap: 20px;
    border-left: 1px solid rgba(0, 0, 0, 0.08);
    overflow-y: auto;
}

.concept-card,
.insight-card,
.fact-box {
    background: #fff;
    border-radius: 16px;
    padding: 20px;
    box-shadow: var(--shadow-sm);
}

.concept-card {
    border-left: 4px solid var(--infinite-blue);
}

.card-header {
    display: flex;
    align-items: center;
    gap: 10px;
    margin-bottom: 12px;
}

.card-icon {
    font-size: 1.4rem;
}

.card-header h4 {
    font-size: 1.1rem;
}

.math-def {
    font-family: var(--font-mono);
    background: rgba(0, 0, 0, 0.04);
    padding: 10px;
    border-radius: 8px;
    text-align: center;
    margin-bottom: 12px;
    font-size: 0.95rem;
}

#conceptDesc {
    font-size: 0.9rem;
    color: var(--text-secondary);
    line-height: 1.6;
}

.insight-card {
    background: linear-gradient(135deg, #fff5f5 0%, #fff 100%);
    border-left: 4px solid var(--boundless-red);
}

.insight-card h4 {
    color: var(--boundless-red);
    margin-bottom: 10px;
}

.insight-card p {
    font-size: 0.9rem;
    color: var(--text-secondary);
    line-height: 1.6;
}

.fact-box {
    border-left: 4px solid var(--aleph-gold);
}

.fact-box h4 {
    color: var(--aleph-gold);
    margin-bottom: 12px;
}

.fact-box ul {
    list-style: none;
    padding: 0;
}

.fact-box li {
    padding: 8px 0;
    font-family: var(--font-mono);
    font-size: 0.85rem;
    color: var(--text-primary);
    border-bottom: 1px solid rgba(0, 0, 0, 0.05);
}

.fact-box li:last-child {
    border-bottom: none;
}