/* ============================================================
   Elasticity — Black & White Glassmorphic Theme
   - Pure white background, black ink, frosted white glass plates
   - Many dynamic SVG illustrations (monochrome)
   - Theme toggle inverts to black bg / white ink (still B&W)
   ============================================================ */

:root {
    --bg: #ffffff;
    --bg-2: #f4f4f4;

    --text: #0a0a0a;
    --text-muted: #555555;
    --text-faint: #9a9a9a;

    --line: rgba(0, 0, 0, .12);
    --line-strong: rgba(0, 0, 0, .30);

    --glass: rgba(255, 255, 255, .55);
    --glass-2: rgba(255, 255, 255, .70);
    --glass-strong: rgba(255, 255, 255, .85);
    --glass-border: rgba(0, 0, 0, .10);
    --glass-border-strong: rgba(0, 0, 0, .26);
    --glass-shadow: 0 8px 30px rgba(0, 0, 0, .08), inset 0 1px 0 rgba(255, 255, 255, .6);

    --dot: rgba(0, 0, 0, .42);

    --radius: 20px;
    --radius-sm: 13px;
    --header-h: 66px;
    --ease: cubic-bezier(.4, 0, .2, 1);
    --t: .3s var(--ease);
}

/* inverted — still strictly black & white */
[data-theme="dark"] {
    --bg: #0a0a0a;
    --bg-2: #151515;
    --text: #f5f5f5;
    --text-muted: #b0b0b0;
    --text-faint: #777777;
    --line: rgba(255, 255, 255, .14);
    --line-strong: rgba(255, 255, 255, .34);
    --glass: rgba(255, 255, 255, .06);
    --glass-2: rgba(255, 255, 255, .09);
    --glass-strong: rgba(255, 255, 255, .14);
    --glass-border: rgba(255, 255, 255, .16);
    --glass-border-strong: rgba(255, 255, 255, .32);
    --glass-shadow: 0 8px 30px rgba(0, 0, 0, .5), inset 0 1px 0 rgba(255, 255, 255, .08);
    --dot: rgba(255, 255, 255, .5);
}

* {
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
    scroll-padding-top: calc(var(--header-h) + 18px);
}

body {
    margin: 0;
    font-family: 'Plus Jakarta Sans', 'Inter', system-ui, -apple-system, 'Segoe UI', Roboto, sans-serif;
    color: var(--text);
    line-height: 1.65;
    background: var(--bg);
    overflow-x: hidden;
    -webkit-font-smoothing: antialiased;
    transition: background .4s var(--ease), color .4s var(--ease);
}

h1,
h2,
h3,
h4 {
    font-family: 'Space Grotesk', 'Plus Jakarta Sans', sans-serif;
    line-height: 1.2;
    color: var(--text);
    letter-spacing: -.01em;
}

a {
    color: var(--text);
}

strong {
    color: var(--text);
    font-weight: 700;
}

/* ============================================================
   SVG illustration helper classes (theme-aware monochrome)
   ============================================================ */
.ink-fill {
    fill: var(--text);
}

.ink-stroke {
    stroke: var(--text);
    fill: none;
}

.gray-stroke {
    stroke: var(--text-muted);
    fill: none;
}

.gray-dash {
    stroke: var(--text-faint);
    fill: none;
}

.paper-fill {
    fill: var(--bg);
}

.gray-fill {
    fill: var(--text-muted);
}

/* grayscale shading layers for pseudo-3D illustrations */
.shade-1 {
    fill: var(--text);
    fill-opacity: .07;
}

.shade-2 {
    fill: var(--text);
    fill-opacity: .14;
}

.shade-3 {
    fill: var(--text);
    fill-opacity: .26;
}

.illus-label {
    fill: var(--text);
    font: 600 13px 'Plus Jakarta Sans', sans-serif;
}

.dgm-stroke {
    stroke: var(--text-muted);
}

.dgm-fill {
    fill: var(--text);
}

.dgm-label {
    fill: var(--text);
    font: 600 14px 'Plus Jakarta Sans', sans-serif;
}

.dgm-label.sm {
    font-size: 12px;
    fill: var(--text-muted);
}

/* ============================================================
   ANIMATED BACKGROUND (monochrome)
   ============================================================ */
.bg-scene {
    position: fixed;
    inset: 0;
    z-index: -2;
    overflow: hidden;
    pointer-events: none;
    background: var(--bg);
}

.bg-svg {
    position: absolute;
    top: 50%;
    left: 50%;
    width: min(150vw, 1500px);
    height: min(150vw, 1500px);
    transform: translate(-50%, -50%);
    opacity: .55;
}

.bg-particles {
    position: absolute;
    inset: 0;
}

.p {
    position: absolute;
    width: 4px;
    height: 4px;
    border-radius: 50%;
    background: var(--dot);
    animation: twinkle var(--dur, 5s) ease-in-out infinite;
    animation-delay: var(--delay, 0s);
}

.orbiter {
    position: absolute;
    top: var(--y, 40%);
    left: var(--x, 40%);
    width: var(--size, 180px);
    height: var(--size, 180px);
    margin: calc(var(--size, 180px) / -2) 0 0 calc(var(--size, 180px) / -2);
    border-radius: 50%;
    animation: spin var(--dur, 18s) linear infinite;
}

.orbiter.rev {
    animation-direction: reverse;
}

.orbiter::after {
    content: "";
    position: absolute;
    top: -3px;
    left: 50%;
    width: 6px;
    height: 6px;
    margin-left: -3px;
    border-radius: 50%;
    background: var(--dot);
}

/* ============================================================
   HEADER
   ============================================================ */
.site-header {
    position: sticky;
    top: 0;
    z-index: 100;
    height: var(--header-h);
    display: flex;
    align-items: center;
    gap: 14px;
    padding: 0 clamp(16px, 4vw, 40px);
    background: var(--glass);
    backdrop-filter: blur(20px) saturate(120%);
    -webkit-backdrop-filter: blur(20px) saturate(120%);
    border-bottom: 1px solid var(--glass-border);
}

.brand {
    display: flex;
    align-items: center;
    gap: 13px;
    font-weight: 800;
    font-size: 1.05rem;
}

.brand .logo {
    position: relative;
    width: 42px;
    height: 42px;
    flex: 0 0 42px;
    display: grid;
    place-items: center;
    border-radius: 50%;
    background: var(--glass-2);
    border: 1px solid var(--glass-border-strong);
}

.brand .logo svg {
    width: 22px;
    height: 22px;
    color: var(--text);
    animation: spin 10s linear infinite;
}

.brand .logo i {
    position: absolute;
    inset: -5px;
    border-radius: 50%;
    border: 1px dashed var(--line-strong);
    animation: spin 6s linear infinite;
}

.brand .logo i::after {
    content: "";
    position: absolute;
    top: -3px;
    left: 50%;
    width: 7px;
    height: 7px;
    margin-left: -3.5px;
    border-radius: 50%;
    background: var(--text);
}

.brand small {
    display: block;
    font-size: .72rem;
    font-weight: 500;
    color: var(--text-muted);
    letter-spacing: .02em;
}

.header-actions {
    margin-left: auto;
    display: flex;
    align-items: center;
    gap: 10px;
}

.icon-btn {
    width: 42px;
    height: 42px;
    display: grid;
    place-items: center;
    border: 1px solid var(--glass-border-strong);
    background: var(--glass);
    backdrop-filter: blur(10px);
    color: var(--text);
    border-radius: 13px;
    cursor: pointer;
    transition: var(--t);
}

.icon-btn:hover {
    transform: translateY(-2px) rotate(-8deg);
    border-color: var(--text);
}

.icon-btn svg {
    width: 20px;
    height: 20px;
}

/* progress bar */
.progress-wrap {
    position: sticky;
    top: var(--header-h);
    z-index: 60;
    height: 3px;
    background: var(--line);
}

.progress-bar {
    position: relative;
    height: 100%;
    width: 0;
    background: var(--text);
    transition: width .5s var(--ease);
}

.progress-bar::after {
    content: "";
    position: absolute;
    right: -3px;
    top: 50%;
    width: 9px;
    height: 9px;
    margin-top: -4.5px;
    border-radius: 50%;
    background: var(--text);
    box-shadow: 0 0 0 3px var(--bg);
}

/* ============================================================
   LAYOUT (no sidebar — single centred column)
   ============================================================ */
main {
    width: 100%;
    max-width: 1080px;
    margin: 0 auto;
    padding: 0 clamp(16px, 4vw, 40px) 90px;
}

/* ============================================================
   HERO
   ============================================================ */
.hero {
    padding: 54px 0 22px;
}

.hero-inner {
    display: flex;
    align-items: center;
    gap: 36px;
    flex-wrap: wrap;
}

.hero-copy {
    flex: 1 1 360px;
    min-width: 280px;
}

.eyebrow {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    font-size: .74rem;
    font-weight: 700;
    letter-spacing: .16em;
    text-transform: uppercase;
    color: var(--text);
    background: var(--glass);
    border: 1px solid var(--glass-border-strong);
    backdrop-filter: blur(10px);
    padding: 6px 14px;
    border-radius: 999px;
    margin-bottom: 18px;
}

.eyebrow::before {
    content: "";
    width: 7px;
    height: 7px;
    border-radius: 50%;
    background: var(--text);
    animation: pulse 1.8s ease-in-out infinite;
}

.hero h1 {
    font-size: clamp(2.4rem, 6vw, 4.1rem);
    font-weight: 900;
    margin: 0 0 14px;
    letter-spacing: -.035em;
}

.hero h1 .spin-word {
    position: relative;
    display: inline-block;
    border-bottom: 4px solid var(--text);
    padding-bottom: 2px;
}

.hero p {
    font-size: 1.06rem;
    color: var(--text-muted);
    max-width: 56ch;
    margin: 0;
}

.hero-illus {
    flex: 0 0 320px;
    width: 320px;
    height: 320px;
    margin: 0 auto;
}

/* stat cards */
.stat-row {
    display: flex;
    flex-wrap: wrap;
    gap: 14px;
    margin-top: 34px;
}

.stat {
    flex: 1 1 130px;
    padding: 16px 18px;
    border-radius: var(--radius);
    background: var(--glass);
    backdrop-filter: blur(16px) saturate(120%);
    -webkit-backdrop-filter: blur(16px) saturate(120%);
    border: 1px solid var(--glass-border);
    box-shadow: var(--glass-shadow);
    transition: var(--t);
}

.stat:hover {
    transform: translateY(-4px);
    border-color: var(--glass-border-strong);
}

.stat .big {
    font-size: 1.8rem;
    font-weight: 800;
    color: var(--text);
    line-height: 1;
}

.stat .lbl {
    font-size: .82rem;
    color: var(--text-muted);
    margin-top: 5px;
}

/* ============================================================
   FILTER BAR
   ============================================================ */
.filter-bar {
    position: sticky;
    top: calc(var(--header-h) + 6px);
    z-index: 50;
    display: flex;
    align-items: center;
    flex-wrap: wrap;
    gap: 10px;
    padding: 12px 16px;
    margin: 26px 0 4px;
    border-radius: var(--radius);
    background: var(--glass-2);
    backdrop-filter: blur(20px) saturate(120%);
    -webkit-backdrop-filter: blur(20px) saturate(120%);
    border: 1px solid var(--glass-border);
    box-shadow: var(--glass-shadow);
}

.filter-bar .lbl {
    font-size: .82rem;
    font-weight: 700;
    color: var(--text-muted);
    display: flex;
    align-items: center;
    gap: 7px;
}

.filter-bar .lbl svg {
    width: 16px;
    height: 16px;
}

.filter-pill {
    border: 1px solid var(--glass-border-strong);
    background: var(--glass);
    backdrop-filter: blur(8px);
    color: var(--text-muted);
    font-weight: 600;
    font-size: .85rem;
    padding: 6px 15px;
    border-radius: 999px;
    cursor: pointer;
    transition: var(--t);
}

.filter-pill:hover {
    color: var(--text);
    border-color: var(--text);
    transform: translateY(-2px);
}

.filter-pill.active {
    background: var(--text);
    border-color: var(--text);
    color: var(--bg);
}

.filter-bar .spacer {
    margin-left: auto;
}

.score-chip {
    font-size: .85rem;
    font-weight: 700;
    color: var(--text);
    background: var(--glass-2);
    padding: 6px 14px;
    border-radius: 999px;
    border: 1px solid var(--glass-border-strong);
}

.btn {
    border: 1px solid var(--glass-border-strong);
    background: var(--glass);
    backdrop-filter: blur(8px);
    color: var(--text);
    font-weight: 600;
    font-size: .85rem;
    padding: 7px 14px;
    border-radius: 11px;
    cursor: pointer;
    display: inline-flex;
    align-items: center;
    gap: 6px;
    transition: var(--t);
}

.btn:hover {
    border-color: var(--text);
    transform: translateY(-2px);
}

.btn:hover svg {
    animation: spin 1s linear;
}

.btn svg {
    width: 15px;
    height: 15px;
}

/* ============================================================
   SECTIONS
   ============================================================ */
.section {
    padding: 46px 0;
    scroll-margin-top: calc(var(--header-h) + 22px);
}

.section-head {
    display: flex;
    align-items: center;
    gap: 16px;
    margin-bottom: 24px;
}

.section-head .badge {
    position: relative;
    width: 50px;
    height: 50px;
    flex: 0 0 50px;
    display: grid;
    place-items: center;
    border-radius: 50%;
    font-weight: 800;
    font-size: 1.15rem;
    color: var(--text);
    background: var(--glass-2);
    border: 1px solid var(--glass-border-strong);
    backdrop-filter: blur(10px);
}

.section-head .badge::before {
    content: "";
    position: absolute;
    inset: -7px;
    border-radius: 50%;
    border: 1.5px dashed var(--line-strong);
    animation: spin 9s linear infinite;
}

.section-head .badge i {
    position: absolute;
    inset: -7px;
    border-radius: 50%;
    animation: spin 4.5s linear infinite;
}

.section-head .badge i::after {
    content: "";
    position: absolute;
    top: -3px;
    left: 50%;
    width: 8px;
    height: 8px;
    margin-left: -4px;
    border-radius: 50%;
    background: var(--text);
}

.section-head h2 {
    font-size: clamp(1.45rem, 3.2vw, 2rem);
    font-weight: 800;
    margin: 0;
}

.section-head .sub {
    font-size: .86rem;
    color: var(--text-muted);
    margin-top: 3px;
}

/* glass cards */
.card {
    padding: 24px 26px;
    margin-bottom: 20px;
    border-radius: var(--radius);
    background: var(--glass);
    backdrop-filter: blur(18px) saturate(120%);
    -webkit-backdrop-filter: blur(18px) saturate(120%);
    border: 1px solid var(--glass-border);
    box-shadow: var(--glass-shadow);
    transition: var(--t);
}

.card:hover {
    border-color: var(--glass-border-strong);
    transform: translateY(-3px);
}

.card ul,
.card ol {
    padding-left: 22px;
    margin: 10px 0;
}

.card li {
    margin: 9px 0;
}

.lead {
    font-size: 1.02rem;
    color: var(--text-muted);
}

.formula {
    display: block;
    background: var(--glass-2);
    border: 1px solid var(--glass-border);
    border-left: 3px solid var(--text);
    border-right: 3px solid var(--text);
    border-radius: var(--radius-sm);
    padding: 13px 16px;
    margin: 14px 0;
    overflow-x: auto;
    text-align: center;
}

.callout {
    background: var(--glass-2);
    border: 1px solid var(--glass-border-strong);
    border-left: 3px solid var(--text);
    border-radius: var(--radius-sm);
    padding: 13px 16px;
    margin: 14px 0;
    font-size: .95rem;
}

.callout.tip {
    border-left-style: dashed;
}

/* ============================================================
   DERIVATION BLOCKS
   ============================================================ */
.derivation h4 {
    display: flex;
    align-items: center;
    gap: 10px;
    font-size: 1.05rem;
    font-weight: 700;
    margin: 0 0 12px;
}

.derivation h4::before {
    content: "∴";
    display: grid;
    place-items: center;
    width: 30px;
    height: 30px;
    flex: 0 0 30px;
    border-radius: 50%;
    font-size: 1rem;
    background: var(--text);
    color: var(--bg);
}

.derivation .setup {
    color: var(--text-muted);
    font-size: .96rem;
}

.derivation .step {
    margin: 18px 0 4px;
    font-size: .78rem;
    font-weight: 800;
    letter-spacing: .08em;
    text-transform: uppercase;
    color: var(--text-muted);
    display: flex;
    align-items: center;
    gap: 8px;
}

.derivation .step::after {
    content: "";
    flex: 1;
    height: 1px;
    background: var(--line);
}

.derivation mjx-container[display="true"] {
    margin: 12px 0 !important;
}

.deriv-result {
    background: var(--glass-2);
    border: 1px solid var(--glass-border-strong);
    border-radius: var(--radius-sm);
    padding: 12px 16px;
    margin: 18px auto 6px;
    text-align: center;
    max-width: 580px;
    font-weight: 600;
}

.chip-list {
    display: flex;
    flex-wrap: wrap;
    gap: 9px;
    margin: 12px 0;
}

.chip {
    font-size: .82rem;
    font-weight: 600;
    padding: 5px 13px;
    border-radius: 999px;
    background: var(--glass-2);
    border: 1px solid var(--glass-border-strong);
    color: var(--text);
}

/* illustrations */
.diagram {
    padding: 18px;
    margin: 18px 0;
    text-align: center;
    border-radius: var(--radius);
    background: var(--glass);
    backdrop-filter: blur(14px) saturate(120%);
    -webkit-backdrop-filter: blur(14px) saturate(120%);
    border: 1px solid var(--glass-border);
    box-shadow: var(--glass-shadow);
}

.diagram svg {
    width: 100%;
    max-width: 460px;
    max-height: 360px;
    height: auto;
    display: block;
    margin: 0 auto;
}

.diagram .cap {
    font-size: .82rem;
    color: var(--text-muted);
    margin-top: 10px;
    font-style: italic;
}

.legend {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    gap: 14px;
    margin-top: 10px;
    font-size: .78rem;
    color: var(--text-muted);
}

.legend span {
    display: inline-flex;
    align-items: center;
    gap: 6px;
}

.legend b {
    font-size: 1.05em;
    color: var(--text);
}

/* two illustrations side by side */
.illus-row {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 18px;
}

@media (max-width: 720px) {
    .illus-row {
        grid-template-columns: 1fr;
    }
}

/* ============================================================
   QUESTIONS
   ============================================================ */
.q-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    flex-wrap: wrap;
    gap: 10px;
    margin: 30px 0 16px;
}

.q-header h3 {
    font-size: 1.18rem;
    font-weight: 800;
    margin: 0;
    display: flex;
    align-items: center;
    gap: 9px;
}

.q-header h3 svg {
    width: 20px;
    height: 20px;
    color: var(--text);
    animation: pulse 2.5s ease-in-out infinite;
}

.q-count {
    font-size: .8rem;
    color: var(--text-muted);
    background: var(--glass);
    border: 1px solid var(--glass-border-strong);
    padding: 4px 11px;
    border-radius: 999px;
}

.question {
    padding: 20px 22px;
    margin-bottom: 16px;
    border-radius: var(--radius);
    background: var(--glass);
    backdrop-filter: blur(18px) saturate(120%);
    -webkit-backdrop-filter: blur(18px) saturate(120%);
    border: 1px solid var(--glass-border);
    box-shadow: var(--glass-shadow);
    transition: var(--t);
}

.question:hover {
    transform: translateY(-3px);
    border-color: var(--glass-border-strong);
}

.question.hidden {
    display: none;
}

.q-top {
    display: flex;
    gap: 13px;
    align-items: flex-start;
}

.q-no {
    flex: 0 0 34px;
    width: 34px;
    height: 34px;
    display: grid;
    place-items: center;
    border-radius: 50%;
    background: var(--text);
    color: var(--bg);
    font-weight: 800;
    font-size: .9rem;
}

.q-text {
    flex: 1;
    font-weight: 600;
    font-size: 1rem;
    margin: 4px 0 0;
}

.q-tags {
    display: flex;
    flex-wrap: wrap;
    gap: 7px;
    margin: 13px 0 15px;
    padding-left: 47px;
}

.tag {
    font-size: .72rem;
    font-weight: 700;
    letter-spacing: .02em;
    padding: 3px 10px;
    border-radius: 8px;
    display: inline-flex;
    align-items: center;
    gap: 4px;
}

.tag svg {
    width: 12px;
    height: 12px;
}

.tag.year {
    background: var(--text);
    color: var(--bg);
}

.options {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 11px;
    padding-left: 47px;
}

@media (max-width:560px) {
    .options {
        grid-template-columns: 1fr;
    }
}

.option {
    display: flex;
    align-items: center;
    gap: 11px;
    padding: 12px 15px;
    border: 1px solid var(--glass-border-strong);
    border-radius: var(--radius-sm);
    background: var(--glass);
    backdrop-filter: blur(8px);
    cursor: pointer;
    font-size: .95rem;
    transition: var(--t);
    text-align: left;
    color: var(--text);
    width: 100%;
}

.option:hover:not(:disabled) {
    border-color: var(--text);
    transform: translateY(-2px);
}

.option .key {
    flex: 0 0 27px;
    width: 27px;
    height: 27px;
    display: grid;
    place-items: center;
    border-radius: 8px;
    background: var(--glass-2);
    border: 1px solid var(--glass-border-strong);
    color: var(--text);
    font-weight: 700;
    font-size: .82rem;
    transition: var(--t);
}

.option:disabled {
    cursor: default;
}

/* correct = solid black fill (B&W) */
.option.correct {
    border-color: var(--text);
    background: var(--text);
    color: var(--bg);
    animation: popIn .4s var(--ease);
}

.option.correct .key {
    background: var(--bg);
    border-color: var(--bg);
    color: var(--text);
}

/* incorrect = outlined, dashed, muted (B&W) */
.option.incorrect {
    border: 2px dashed var(--text);
    background: var(--glass-2);
    animation: shake .4s var(--ease);
}

.option.incorrect .otext {
    text-decoration: line-through;
    opacity: .7;
}

.option .mark {
    margin-left: auto;
    font-weight: 800;
    opacity: 0;
    transition: var(--t);
}

.option.correct .mark,
.option.incorrect .mark {
    opacity: 1;
}

.explanation {
    margin: 15px 0 2px 47px;
    padding: 14px 16px;
    background: var(--glass-2);
    border: 1px solid var(--glass-border);
    border-left: 3px solid var(--text);
    border-radius: var(--radius-sm);
    font-size: .92rem;
    color: var(--text-muted);
    display: none;
}

.explanation.show {
    display: block;
    animation: fadeUp .4s var(--ease);
}

.explanation b {
    color: var(--text);
}

.no-result {
    text-align: center;
    color: var(--text-muted);
    padding: 26px;
    font-style: italic;
    display: none;
}

/* ============================================================
   HIGH YIELD
   ============================================================ */
.hy-grid {
    display: grid;
    gap: 12px;
}

.hy-item {
    display: flex;
    gap: 13px;
    align-items: flex-start;
    padding: 14px 17px;
    border-radius: var(--radius-sm);
    background: var(--glass);
    backdrop-filter: blur(14px) saturate(120%);
    -webkit-backdrop-filter: blur(14px) saturate(120%);
    border: 1px solid var(--glass-border);
    box-shadow: var(--glass-shadow);
    transition: var(--t);
}

.hy-item:hover {
    transform: translateX(6px);
    border-color: var(--glass-border-strong);
}

.hy-item .dot {
    flex: 0 0 10px;
    width: 10px;
    height: 10px;
    margin-top: 7px;
    border-radius: 50%;
    background: var(--text);
    animation: pulse 2.4s ease-in-out infinite;
}

/* ============================================================
   FOOTER + TO TOP
   ============================================================ */
.site-footer {
    max-width: 1080px;
    margin: 0 auto 18px;
    padding: 28px;
    text-align: center;
    color: var(--text-muted);
    font-size: .88rem;
    border-radius: var(--radius);
    background: var(--glass);
    backdrop-filter: blur(14px) saturate(120%);
    -webkit-backdrop-filter: blur(14px) saturate(120%);
    border: 1px solid var(--glass-border);
    box-shadow: var(--glass-shadow);
}

.to-top {
    position: fixed;
    right: 22px;
    bottom: 22px;
    width: 50px;
    height: 50px;
    border-radius: 50%;
    border: 1px solid var(--glass-border-strong);
    background: var(--glass-strong);
    backdrop-filter: blur(14px);
    color: var(--text);
    cursor: pointer;
    display: grid;
    place-items: center;
    box-shadow: var(--glass-shadow);
    opacity: 0;
    pointer-events: none;
    transform: translateY(14px);
    transition: var(--t);
    z-index: 90;
    animation: float 3s ease-in-out infinite;
}

.to-top.show {
    opacity: 1;
    pointer-events: auto;
    transform: translateY(0);
}

.to-top:hover {
    border-color: var(--text);
    transform: translateY(-4px) scale(1.05);
}

.to-top svg {
    width: 22px;
    height: 22px;
}

/* scroll reveal */
.reveal {
    opacity: 0;
    transform: translateY(28px);
}

.reveal.in {
    opacity: 1;
    transform: none;
    transition: opacity .7s var(--ease), transform .7s var(--ease);
}

/* ============================================================
   KEYFRAMES
   ============================================================ */
@keyframes spin {
    to {
        transform: rotate(360deg);
    }
}

@keyframes pulse {

    0%,
    100% {
        transform: scale(1);
        opacity: 1;
    }

    50% {
        transform: scale(1.18);
        opacity: .7;
    }
}

@keyframes float {

    0%,
    100% {
        transform: translateY(0);
    }

    50% {
        transform: translateY(-8px);
    }
}

@keyframes twinkle {

    0%,
    100% {
        opacity: .12;
        transform: scale(.7);
    }

    50% {
        opacity: .9;
        transform: scale(1.2);
    }
}

@keyframes fadeUp {
    from {
        opacity: 0;
        transform: translateY(10px);
    }

    to {
        opacity: 1;
        transform: translateY(0);
    }
}

@keyframes popIn {
    0% {
        transform: scale(.94);
    }

    55% {
        transform: scale(1.03);
    }

    100% {
        transform: scale(1);
    }
}

@keyframes shake {

    0%,
    100% {
        transform: translateX(0);
    }

    25% {
        transform: translateX(-5px);
    }

    75% {
        transform: translateX(5px);
    }
}

/* ============================================================
   RESPONSIVE
   ============================================================ */
@media (max-width: 560px) {

    .q-tags,
    .options,
    .q-actions {
        padding-left: 0;
    }

    .explanation {
        margin-left: 0;
    }

    .hero-illus {
        flex-basis: 250px;
        width: 250px;
        height: 250px;
    }
}

@media (prefers-reduced-motion: reduce) {
    * {
        animation-duration: .001ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: .1ms !important;
    }
}