:root {
    --c-bg: #09090d;
    --c-surface: #101117;
    --c-surface-hover: #1a1b24;
    --c-border: rgba(244, 244, 245, 0.08);
    --c-border-strong: rgba(244, 244, 245, 0.14);
    --c-border-hover: rgba(244, 244, 245, 0.26);
    --c-text: #f4f4f5;
    --c-text-muted: #a1a1aa;
    --c-text-faint: #6f6f78;
    --c-primary: #818cf8;
    --c-primary-2: #c084fc;
    --c-on-primary: #0b0b14;
    --c-primary-tint: color-mix(in srgb, var(--c-primary) 13%, transparent);
    --c-primary-tint-strong: color-mix(in srgb, var(--c-primary) 22%, transparent);
    --c-primary-border: color-mix(in srgb, var(--c-primary) 34%, transparent);
    --c-primary-glow: color-mix(in srgb, var(--c-primary) 24%, transparent);
    --c-focus-ring: color-mix(in srgb, var(--c-primary) 55%, transparent);
    --c-sheen: linear-gradient(180deg, rgba(255, 255, 255, 0.035), transparent 46%);
    --font-sans: "Inter", ui-sans-serif, system-ui, sans-serif;
    --font-display: "Space Grotesk", "Inter", ui-sans-serif, sans-serif;
    --font-serif: "Instrument Serif", Georgia, serif;
    --font-mono: "JetBrains Mono", ui-monospace, monospace;
    --space-1: 0.25rem;
    --space-2: 0.5rem;
    --space-3: 0.75rem;
    --space-4: 1rem;
    --space-5: 1.25rem;
    --space-6: 1.5rem;
    --space-8: 2rem;
    --radius-sm: 0.5rem;
    --radius-md: 0.75rem;
    --radius-lg: 1rem;
}

:root[data-mode="light"] {
    --c-bg: #f7f7f8;
    --c-surface: #ffffff;
    --c-surface-hover: #efeff2;
    --c-border: rgba(9, 9, 11, 0.09);
    --c-border-strong: rgba(9, 9, 11, 0.16);
    --c-border-hover: rgba(9, 9, 11, 0.3);
    --c-text: #18181b;
    --c-text-muted: #52525b;
    --c-text-faint: #8e8e97;
    --c-primary: #4f46e5;
    --c-primary-2: #9333ea;
    --c-on-primary: #ffffff;
}

.ref-nav .container-xl,
.rfs-nav-inner {
    display: flex;
    align-items: center;
    gap: var(--space-3);
    flex-wrap: wrap;
}

.rfs-nav {
    position: sticky;
    top: 0;
    z-index: 1040;
    background: color-mix(in srgb, var(--c-bg) 82%, transparent);
    -webkit-backdrop-filter: blur(12px);
    backdrop-filter: blur(12px);
    border-bottom: 1px solid var(--c-border);
    padding: 0.5rem 0;
}

.rfs-brand {
    display: inline-flex;
    align-items: center;
    gap: var(--space-2);
    color: var(--c-text);
    font-family: var(--font-display);
    font-weight: 600;
    letter-spacing: -0.01em;
    text-decoration: none;
    margin-right: var(--space-2);
}

.rfs-brand-dot {
    width: 8px;
    height: 8px;
    border-radius: 2px;
    background: linear-gradient(135deg, var(--c-primary), var(--c-primary-2));
    box-shadow: 0 0 12px var(--c-primary-glow);
}

.rfs-links {
    display: flex;
    align-items: center;
    gap: var(--space-1);
    flex-wrap: wrap;
}

.rfs-links a {
    font-size: 0.8rem;
}

.rfs-actions {
    display: flex;
    align-items: center;
    gap: var(--space-2);
    margin-left: auto;
}

.rfs-actions #theme-btn {
    width: 2.25rem;
    padding: 0;
}

.rfs-hero {
    position: relative;
    overflow: hidden;
    padding-block: clamp(2.5rem, 6vw, 4.5rem);
    border-bottom: 1px solid var(--c-border);
}

.rfs-hero-inner {
    display: grid;
    grid-template-columns: minmax(0, 1.4fr) minmax(0, 0.8fr);
    gap: clamp(2rem, 5vw, 4rem);
    align-items: center;
}

.rfs-eyebrow {
    display: flex;
    align-items: center;
    gap: var(--space-2);
    margin-bottom: var(--space-4);
    color: var(--c-text-muted);
    font-family: var(--font-mono);
    font-size: 0.72rem;
    letter-spacing: 0.08em;
    text-transform: uppercase;
}

.rfs-eyebrow::before {
    content: "";
    width: 22px;
    height: 1px;
    background: var(--c-primary-border);
}

.rfs-hero h1 {
    margin: 0 0 var(--space-4);
    font-family: var(--font-display);
    font-size: clamp(2.25rem, 1.5rem + 4vw, 4.25rem);
    font-weight: 600;
    line-height: 1.02;
    letter-spacing: -0.03em;
}

.rfs-hero p {
    max-width: 58ch;
    color: var(--c-text-muted);
    font-size: 1.0625rem;
    line-height: 1.7;
    margin-bottom: var(--space-5);
}

.rfs-hero-cta {
    display: flex;
    flex-wrap: wrap;
    gap: var(--space-3);
}

.rfs-bento {
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: var(--space-3);
}

.rfs-stat {
    background: var(--c-surface);
    background-image: var(--c-sheen);
    border: 1px solid var(--c-border);
    border-radius: var(--radius-lg);
    padding: var(--space-4);
    min-width: 0;
}

.rfs-stat--wide {
    grid-column: span 2;
}

.rfs-stat-num {
    display: block;
    font-family: var(--font-display);
    font-size: clamp(2rem, 1.2rem + 2.4vw, 3rem);
    font-weight: 600;
    letter-spacing: -0.03em;
    line-height: 1;
    margin-bottom: var(--space-2);
}

.rfs-stat-label {
    color: var(--c-text-faint);
    font-size: 0.82rem;
    line-height: 1.45;
}

@media (max-width: 992px) {
    .rfs-hero-inner {
        grid-template-columns: 1fr;
    }

    .rfs-bento {
        max-width: 34rem;
    }
}

@media (max-width: 640px) {
    .rfs-links {
        display: none;
    }
}

.sec-head h2 {
    font-family: var(--font-display);
    font-size: clamp(1.5rem, 1.15rem + 1.6vw, 2.1rem);
    font-weight: 600;
    letter-spacing: -0.02em;
}

.sec-lead {
    color: var(--c-text-muted);
}

.gloss-card {
    background: var(--c-surface);
    background-image: var(--c-sheen);
    border: 1px solid var(--c-border);
    border-radius: var(--radius-md);
}

.gloss-card:hover {
    border-color: var(--c-border-strong);
    box-shadow: none;
}

.gloss-term {
    font-family: var(--font-display);
    font-weight: 600;
}

.gloss-body {
    background: color-mix(in srgb, var(--c-surface-2, var(--c-surface)) 90%, transparent);
    border-top: 1px solid var(--c-border);
}

.cat-group-header {
    font-family: var(--font-mono);
    letter-spacing: 0.12em;
}

.alpha-btn {
    background: var(--c-surface);
    border-color: var(--c-border-strong);
    color: var(--c-text-muted);
}

.alpha-btn:hover {
    border-color: var(--c-primary-border);
    color: var(--c-primary);
}

.q-item {
    background: var(--c-surface);
    background-image: var(--c-sheen);
    border: 1px solid var(--c-border);
    border-radius: var(--radius-md);
}

.q-item-header {
    color: var(--c-text);
}

.q-item-body {
    background: color-mix(in srgb, var(--c-surface-2, var(--c-surface)) 90%, transparent);
    border-top: 1px solid var(--c-border);
    color: var(--c-text-muted);
    font-size: 0.92rem;
    line-height: 1.75;
}

.q-option.correct {
    background: color-mix(in srgb, #34d399 10%, transparent);
    border: 1px solid color-mix(in srgb, #34d399 30%, transparent);
}

.q-option.wrong {
    background: color-mix(in srgb, #f87171 7%, transparent);
    border: 1px solid color-mix(in srgb, #f87171 20%, transparent);
}

.icb-item {
    color: var(--c-text-muted);
    font-size: 0.8rem;
}

.icb-item:hover {
    background: var(--c-surface-hover);
    color: var(--c-text);
}

.sticky-search {
    background: var(--c-bg);
    border-bottom: 1px solid var(--c-border);
}

.form-control,
.form-select {
    background-color: var(--c-surface);
    border-color: var(--c-border-strong);
    color: var(--c-text);
}

.form-control:focus,
.form-select:focus {
    background-color: var(--c-surface);
    border-color: var(--c-primary-border);
    box-shadow: 0 0 0 3px var(--c-focus-ring);
    color: var(--c-text);
}

.input-group-text {
    background: var(--c-surface);
    border-color: var(--c-border-strong);
    color: var(--c-text-faint);
}

#theme-btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
}

.toc-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(min(100%, 13rem), 1fr));
    gap: var(--space-3);
}

.toc-tile {
    background: var(--c-surface);
    background-image: var(--c-sheen);
    border: 1px solid var(--c-border);
    border-radius: var(--radius-md);
}

.toc-tile:hover {
    border-color: var(--c-primary-border);
    box-shadow: none;
}

.toc-ico {
    color: var(--c-primary);
    background: var(--c-primary-tint);
}

.model-card.card {
    background: var(--c-surface);
    background-image: var(--c-sheen);
    border: 1px solid var(--c-border);
    border-radius: var(--radius-lg);
}

.formula-box {
    background: var(--c-surface-2, var(--c-surface));
    border-color: var(--c-border-strong);
    color: var(--c-text);
}

.doz-panel {
    background: color-mix(in srgb, #fbbf24 7%, var(--c-surface));
    border-color: var(--c-warning-border, rgba(251, 191, 36, 0.3));
}

.doz-action {
    background: var(--c-surface);
    border-color: var(--c-border);
}

.doz-action:hover {
    border-color: #fbbf24;
}

.si {
    width: 1em;
    height: 1em;
    fill: none;
    stroke: currentColor;
    stroke-width: 1.8;
    stroke-linecap: round;
    stroke-linejoin: round;
    vertical-align: -0.12em;
    flex-shrink: 0;
}

.si--lg {
    width: 1.35em;
    height: 1.35em;
}

.rfs-quick {
    padding-block: var(--space-8) var(--space-2);
}

.rfs-quick-head {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: var(--space-4);
}

.rfs-quick-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(min(100%, 13rem), 1fr));
    gap: var(--space-3);
}

.rfs-q {
    display: flex;
    flex-direction: column;
    gap: var(--space-1);
    min-width: 0;
    padding: var(--space-4);
    background: var(--c-surface);
    background-image: var(--c-sheen);
    border: 1px solid var(--c-border);
    border-radius: var(--radius-md);
    color: var(--c-text);
    text-decoration: none;
    transition: border-color 0.15s ease, background-color 0.15s ease;
}

.rfs-q:hover {
    border-color: var(--c-primary-border);
    background-color: var(--c-surface-hover);
}

.rfs-q-ico {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 2rem;
    height: 2rem;
    border-radius: var(--radius-sm);
    background: var(--c-primary-tint);
    color: var(--c-primary);
    margin-bottom: var(--space-2);
}

.rfs-q-tt {
    font-family: var(--font-display);
    font-weight: 600;
    font-size: 0.95rem;
    letter-spacing: -0.01em;
    color: var(--c-text);
}

.rfs-q-sub {
    color: var(--c-text-faint);
    font-size: 0.78rem;
    line-height: 1.4;
}

#theme-ic {
    pointer-events: none;
}

.container {
    width: 100%;
    max-width: 76rem;
    margin-inline: auto;
    padding-inline: clamp(1rem, 4vw, 2rem);
}

.serif-i {
    font-family: var(--font-serif);
    font-style: italic;
    font-weight: 400;
    letter-spacing: -0.01em;
}

.tag {
    display: inline-flex;
    align-items: center;
    gap: 0.375rem;
    padding: 0.125rem 0.5rem;
    border: 1px solid var(--c-border-strong);
    border-radius: 999px;
    background: transparent;
    color: var(--c-text-muted);
    font-family: var(--font-mono);
    font-size: 0.6875rem;
    font-weight: 500;
    line-height: 1.6;
    letter-spacing: 0.08em;
    text-transform: uppercase;
    white-space: nowrap;
}

.tag--accent {
    color: var(--c-primary);
    border-color: var(--c-primary-border);
    background: var(--c-primary-tint);
}

.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 0.375rem;
    min-height: 2.25rem;
    padding: 0.375rem 0.9375rem;
    border: 1px solid transparent;
    border-radius: 0.5rem;
    background: transparent;
    color: var(--c-text);
    font-family: var(--font-sans);
    font-size: 0.875rem;
    font-weight: 600;
    line-height: 1.2;
    text-decoration: none;
    white-space: nowrap;
    cursor: pointer;
    -webkit-tap-highlight-color: transparent;
    transition: background-color 0.15s ease, border-color 0.15s ease, color 0.15s ease;
}

.btn--sm {
    min-height: 1.875rem;
    padding: 0.1875rem 0.625rem;
    font-size: 0.8125rem;
}

.btn .si {
    width: 1.1em;
    height: 1.1em;
}

.btn--primary {
    background: linear-gradient(135deg, var(--c-primary) 0%, var(--c-primary-2) 110%);
    color: var(--c-on-primary);
    box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.14), 0 8px 24px -14px var(--c-primary-glow);
}

.btn--primary:hover {
    filter: saturate(1.04) brightness(1.05);
}

.btn--outline {
    background: var(--c-surface);
    border-color: var(--c-border-strong);
    color: var(--c-text);
}

.btn--outline:hover {
    background: var(--c-surface-hover);
    border-color: var(--c-border-hover);
}

.btn--ghost {
    color: var(--c-text-muted);
}

.btn--ghost:hover {
    background: var(--c-surface-hover);
    color: var(--c-text);
}

.rfs-hero-inner {
    grid-template-columns: minmax(0, 1.25fr) minmax(0, 0.95fr);
}

.rfs-stats {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
    margin-top: var(--space-5);
}

.rfs-chip {
    display: inline-flex;
    align-items: baseline;
    gap: 0.5rem;
    padding: 0.375rem 0.75rem;
    background: var(--c-surface);
    border: 1px solid var(--c-border);
    border-radius: var(--radius-md);
}

.rfs-chip b {
    font-family: var(--font-display);
    font-size: 1.05rem;
    font-weight: 600;
    letter-spacing: -0.01em;
    color: var(--c-text);
}

.rfs-chip span {
    color: var(--c-text-faint);
    font-size: 0.72rem;
}

.rfs-qpanel {
    background: var(--c-surface);
    background-image: var(--c-sheen);
    border: 1px solid var(--c-border);
    border-radius: var(--radius-lg);
    padding: var(--space-3);
}

.rfs-qpanel-head {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: var(--space-2);
    padding: 0.375rem 0.5rem 0.625rem;
}

.rfs-qrow {
    display: flex;
    align-items: center;
    gap: var(--space-3);
    width: 100%;
    padding: 0.5rem 0.625rem;
    border-radius: var(--radius-sm);
    color: var(--c-text);
    text-decoration: none;
    transition: background-color 0.15s ease;
}

.rfs-qrow:hover {
    background: var(--c-surface-hover);
}

.rfs-qrow + .rfs-qrow {
    margin-top: 0.125rem;
}

.rfs-qrow-ico {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 1.9rem;
    height: 1.9rem;
    flex-shrink: 0;
    border-radius: var(--radius-sm);
    background: var(--c-primary-tint);
    color: var(--c-primary);
}

.rfs-qrow-main {
    flex: 1 1 auto;
    min-width: 0;
    display: flex;
    flex-direction: column;
    line-height: 1.35;
}

.rfs-qrow-tt {
    font-family: var(--font-display);
    font-size: 0.9rem;
    font-weight: 600;
    letter-spacing: -0.01em;
    color: var(--c-text);
}

.rfs-qrow-sub {
    color: var(--c-text-faint);
    font-size: 0.74rem;
}

.rfs-qrow-arrow {
    color: var(--c-text-faint);
    opacity: 0;
    transition: opacity 0.15s ease, transform 0.15s ease, color 0.15s ease;
}

.rfs-qrow:hover .rfs-qrow-arrow {
    opacity: 1;
    transform: translateX(2px);
    color: var(--c-primary);
}

.rfs-qrow--cta {
    margin-top: var(--space-2);
    background: var(--c-primary-tint);
    border: 1px solid var(--c-primary-border);
    border-radius: var(--radius-md);
    padding: 0.625rem;
}

.rfs-qrow--cta:hover {
    background: var(--c-primary-tint-strong);
}

.rfs-qrow--cta .rfs-qrow-tt {
    color: var(--c-primary);
}

.si--arrow {
    width: 1.1em;
    height: 1.1em;
}