:root {
  --ink: #102520;
  --ink-soft: #425851;
  --navy: #0c2528;
  --navy-light: #143639;
  --lime: #c8f135;
  --lime-dark: #a7cf17;
  --mint: #dff3e9;
  --paper: #f5f5ee;
  --white: #ffffff;
  --line: #d8ded7;
  --orange: #ff8a4c;
  --blue: #4a8ded;
  --shadow: 0 24px 60px rgba(22, 49, 42, 0.1);
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  background: var(--paper);
  color: var(--ink);
  font-family: "Noto Sans JP", sans-serif;
  line-height: 1.75;
  overflow-x: hidden;
}

button,
input {
  font: inherit;
}

button,
a {
  -webkit-tap-highlight-color: transparent;
}

a {
  color: inherit;
  text-decoration: none;
}

.page-glow {
  position: fixed;
  z-index: -1;
  border-radius: 50%;
  filter: blur(10px);
  opacity: 0.55;
  pointer-events: none;
}

.page-glow-one {
  width: 460px;
  height: 460px;
  top: -200px;
  right: -140px;
  background: rgba(200, 241, 53, 0.2);
}

.page-glow-two {
  width: 360px;
  height: 360px;
  top: 780px;
  left: -250px;
  background: rgba(87, 192, 156, 0.13);
}

.site-header {
  width: min(1180px, calc(100% - 48px));
  height: 88px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  border-bottom: 1px solid rgba(16, 37, 32, 0.12);
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 12px;
}

.brand-mark {
  width: 40px;
  height: 40px;
  display: grid;
  place-items: center;
  border-radius: 12px;
  background: var(--navy);
  color: var(--lime);
  font: 800 20px/1 "DM Sans", sans-serif;
  transform: rotate(-5deg);
}

.brand > span:last-child {
  display: flex;
  flex-direction: column;
  line-height: 1.2;
}

.brand strong {
  font: 700 16px "DM Sans", sans-serif;
  letter-spacing: 0.08em;
}

.brand small {
  margin-top: 4px;
  color: var(--ink-soft);
  font-size: 10px;
  letter-spacing: 0.08em;
}

.main-nav {
  display: flex;
  gap: 42px;
  font-size: 13px;
  font-weight: 700;
}

.main-nav a {
  position: relative;
}

.main-nav a::after {
  content: "";
  position: absolute;
  left: 0;
  right: 100%;
  bottom: -8px;
  height: 2px;
  background: var(--lime-dark);
  transition: right 0.25s ease;
}

.main-nav a:hover::after {
  right: 0;
}

.progress-pill {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 10px 16px;
  border: 1px solid var(--line);
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.7);
  color: var(--ink);
  cursor: default;
}

.progress-dot {
  width: 9px;
  height: 9px;
  border-radius: 50%;
  background: var(--lime-dark);
  box-shadow: 0 0 0 5px rgba(167, 207, 23, 0.13);
}

.progress-pill span {
  font-size: 12px;
}

.hero {
  width: min(1180px, calc(100% - 48px));
  min-height: 650px;
  margin: 0 auto;
  display: grid;
  grid-template-columns: 0.95fr 1.05fr;
  align-items: center;
  gap: 70px;
  padding: 72px 0 86px;
}

.eyebrow {
  margin: 0 0 18px;
  color: #668078;
  font: 700 11px "DM Sans", sans-serif;
  letter-spacing: 0.18em;
  text-transform: uppercase;
}

.eyebrow span {
  margin-right: 14px;
  padding: 6px 9px;
  border-radius: 4px;
  background: var(--navy);
  color: var(--lime);
}

h1,
h2,
h3,
p {
  margin-top: 0;
}

h1 {
  margin-bottom: 26px;
  font-size: clamp(44px, 5.3vw, 76px);
  line-height: 1.25;
  letter-spacing: -0.06em;
}

h1 em {
  position: relative;
  color: var(--ink);
  font-style: normal;
  z-index: 0;
}

h1 em::after {
  content: "";
  position: absolute;
  z-index: -1;
  left: -2%;
  right: -4%;
  bottom: 5px;
  height: 18px;
  background: var(--lime);
  transform: rotate(-1deg);
}

.hero-description {
  max-width: 500px;
  margin-bottom: 34px;
  color: var(--ink-soft);
  font-size: 16px;
}

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

.button {
  min-height: 52px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 28px;
  padding: 0 24px;
  border: 0;
  border-radius: 8px;
  font-size: 13px;
  font-weight: 700;
  cursor: pointer;
  transition: transform 0.2s ease, box-shadow 0.2s ease, background 0.2s ease;
}

.button:hover:not(:disabled) {
  transform: translateY(-2px);
}

.button-primary {
  background: var(--navy);
  color: var(--white);
  box-shadow: 0 12px 24px rgba(12, 37, 40, 0.18);
}

.button-primary span {
  color: var(--lime);
  font-size: 18px;
}

.button-ghost {
  border: 1px solid var(--line);
  background: transparent;
  color: var(--ink-soft);
}

.hero-stats {
  display: flex;
  gap: 42px;
  margin-top: 52px;
}

.hero-stats div {
  display: flex;
  flex-direction: column;
}

.hero-stats strong {
  font: 700 24px "DM Sans", sans-serif;
}

.hero-stats span {
  color: #75877f;
  font-size: 10px;
  font-weight: 700;
  letter-spacing: 0.08em;
}

.hero-visual {
  position: relative;
  min-height: 490px;
  border-radius: 30px;
  background:
    radial-gradient(circle at center, rgba(200, 241, 53, 0.09), transparent 45%),
    linear-gradient(145deg, #123638, #081d20);
  box-shadow: 0 36px 70px rgba(8, 29, 32, 0.2);
  overflow: hidden;
}

.hero-visual::before {
  content: "";
  position: absolute;
  inset: 0;
  background-image: linear-gradient(rgba(255, 255, 255, 0.04) 1px, transparent 1px),
    linear-gradient(90deg, rgba(255, 255, 255, 0.04) 1px, transparent 1px);
  background-size: 48px 48px;
  mask-image: radial-gradient(circle, black, transparent 75%);
}

.orbit {
  position: absolute;
  top: 50%;
  left: 50%;
  border: 1px solid rgba(200, 241, 53, 0.22);
  border-radius: 50%;
  transform: translate(-50%, -50%) rotate(-10deg);
}

.orbit-one {
  width: 450px;
  height: 210px;
}

.orbit-two {
  width: 370px;
  height: 330px;
  transform: translate(-50%, -50%) rotate(36deg);
}

.field-line {
  position: absolute;
  top: 50%;
  left: 50%;
  width: 300px;
  height: 120px;
  border-top: 1px solid rgba(223, 243, 233, 0.35);
  border-radius: 50%;
  transform-origin: center;
}

.field-line-one {
  transform: translate(-50%, -50%);
}

.field-line-two {
  transform: translate(-50%, -50%) rotate(25deg);
}

.field-line-three {
  transform: translate(-50%, -50%) rotate(-25deg);
}

.charge {
  position: absolute;
  top: 48%;
  width: 112px;
  height: 112px;
  display: grid;
  place-items: center;
  border-radius: 50%;
  color: white;
  box-shadow: inset -12px -16px 30px rgba(0, 0, 0, 0.16), 0 20px 40px rgba(0, 0, 0, 0.28);
  animation: float 4s ease-in-out infinite;
}

.charge span {
  font: 500 42px/1 "DM Sans", sans-serif;
}

.charge small {
  position: absolute;
  bottom: -35px;
  color: rgba(255, 255, 255, 0.72);
  font: 600 12px "DM Sans", sans-serif;
  letter-spacing: 0.12em;
}

.charge-positive {
  left: 18%;
  background: linear-gradient(145deg, #ffaf72, var(--orange));
}

.charge-negative {
  right: 18%;
  background: linear-gradient(145deg, #72b5ff, #3976d8);
  animation-delay: -2s;
}

@keyframes float {
  0%, 100% { transform: translateY(-50%); }
  50% { transform: translateY(calc(-50% - 10px)); }
}

.force-label {
  position: absolute;
  top: 28%;
  padding: 4px 10px;
  border: 1px solid rgba(255, 255, 255, 0.15);
  border-radius: 999px;
  color: rgba(255, 255, 255, 0.65);
  font-size: 9px;
  letter-spacing: 0.12em;
}

.label-left { left: 25%; }
.label-right { right: 26%; }

.visual-caption {
  position: absolute;
  left: 34px;
  right: 34px;
  bottom: 26px;
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  color: white;
}

.visual-caption > span {
  color: rgba(255, 255, 255, 0.45);
  font: 600 9px "DM Sans", sans-serif;
  letter-spacing: 0.18em;
}

.visual-caption strong {
  font: 600 20px "DM Sans", sans-serif;
  letter-spacing: 0.03em;
}

.visual-caption strong span {
  display: inline-block;
  padding-bottom: 2px;
  border-bottom: 1px solid var(--lime);
}

.section {
  padding: 110px max(24px, calc((100% - 1180px) / 2));
}

.learning-section {
  background: var(--white);
}

.section-heading {
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  gap: 60px;
  margin-bottom: 52px;
}

.section-heading h2,
.quiz-intro h2 {
  margin-bottom: 0;
  font-size: clamp(28px, 3.4vw, 43px);
  line-height: 1.35;
  letter-spacing: -0.04em;
}

.section-heading > p {
  max-width: 440px;
  margin-bottom: 5px;
  color: var(--ink-soft);
  font-size: 13px;
}

.learning-layout {
  display: grid;
  grid-template-columns: 360px 1fr;
  gap: 34px;
}

.chapter-list {
  display: flex;
  flex-direction: column;
  gap: 9px;
}

.chapter-button {
  position: relative;
  width: 100%;
  min-height: 86px;
  display: grid;
  grid-template-columns: 42px 1fr 20px;
  align-items: center;
  gap: 12px;
  padding: 14px 16px;
  border: 1px solid transparent;
  border-radius: 12px;
  background: transparent;
  color: var(--ink);
  text-align: left;
  cursor: pointer;
  transition: 0.2s ease;
}

.chapter-button:hover {
  background: var(--paper);
}

.chapter-button.active {
  border-color: #dfe6dd;
  background: #f5f8f1;
  box-shadow: 0 10px 28px rgba(22, 49, 42, 0.06);
}

.chapter-number {
  color: #93a19a;
  font: 600 12px "DM Sans", sans-serif;
}

.chapter-button.active .chapter-number {
  color: var(--lime-dark);
}

.chapter-button > span:nth-child(2) {
  display: flex;
  flex-direction: column;
}

.chapter-button strong {
  font-size: 13px;
}

.chapter-button small {
  margin-top: 3px;
  color: #93a19a;
  font-size: 10px;
}

.chapter-status {
  width: 8px;
  height: 8px;
  border: 1px solid #b7c1bc;
  border-radius: 50%;
}

.chapter-button.completed .chapter-status {
  border-color: var(--lime-dark);
  background: var(--lime-dark);
  box-shadow: 0 0 0 4px rgba(167, 207, 23, 0.12);
}

.lesson-card {
  min-height: 570px;
  padding: 42px 46px;
  border-radius: 20px;
  background: var(--navy);
  color: white;
  box-shadow: var(--shadow);
}

.lesson-topline {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.lesson-topline > span {
  color: var(--lime);
  font: 700 10px "DM Sans", sans-serif;
  letter-spacing: 0.18em;
}

.bookmark-button {
  width: 36px;
  height: 36px;
  border: 1px solid rgba(255, 255, 255, 0.14);
  border-radius: 50%;
  background: transparent;
  color: rgba(255, 255, 255, 0.6);
  font-size: 20px;
  cursor: pointer;
}

.bookmark-button.active {
  color: var(--lime);
}

.lesson-card h3 {
  margin: 16px 0 12px;
  font-size: 28px;
}

.lesson-lead {
  max-width: 650px;
  color: rgba(255, 255, 255, 0.65);
  font-size: 13px;
}

.concept-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 12px;
  margin: 28px 0;
}

.concept-card {
  min-height: 104px;
  padding: 17px;
  border: 1px solid rgba(255, 255, 255, 0.1);
  border-radius: 10px;
  background: rgba(255, 255, 255, 0.045);
}

.concept-card small {
  display: block;
  margin-bottom: 8px;
  color: var(--lime);
  font: 600 9px "DM Sans", sans-serif;
  letter-spacing: 0.12em;
}

.concept-card strong {
  display: block;
  margin-bottom: 5px;
  font-size: 13px;
}

.concept-card p {
  margin: 0;
  color: rgba(255, 255, 255, 0.55);
  font-size: 10px;
  line-height: 1.6;
}

.formula-panel {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 28px;
  align-items: center;
  padding: 20px 24px;
  border-left: 3px solid var(--lime);
  background: rgba(200, 241, 53, 0.07);
}

.formula-panel small {
  color: var(--lime);
  font: 700 9px "DM Sans", sans-serif;
  letter-spacing: 0.15em;
}

.formula-panel > div p {
  margin: 5px 0 0;
  font: 600 25px "DM Sans", sans-serif;
}

.formula-note {
  margin: 0;
  color: rgba(255, 255, 255, 0.58);
  font-size: 10px;
}

.exam-tip {
  display: flex;
  gap: 20px;
  margin-top: 24px;
  padding: 14px 18px;
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.05);
}

.exam-tip span {
  flex: 0 0 auto;
  color: var(--lime);
  font-size: 10px;
  font-weight: 700;
}

.exam-tip p {
  margin: 0;
  color: rgba(255, 255, 255, 0.66);
  font-size: 11px;
}

.chapter-practice {
  margin-top: 24px;
  padding: 22px;
  border: 1px solid rgba(255, 255, 255, 0.1);
  border-radius: 14px;
  background: rgba(255, 255, 255, 0.04);
}

.practice-heading {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 18px;
  margin-bottom: 16px;
}

.practice-heading small {
  color: var(--lime);
  font: 700 9px "DM Sans", sans-serif;
  letter-spacing: 0.15em;
}

.practice-heading h4 {
  margin: 3px 0 0;
  font-size: 16px;
}

.practice-heading p {
  min-width: 54px;
  margin: 0;
  padding: 4px 10px;
  border-radius: 999px;
  background: rgba(200, 241, 53, 0.1);
  color: var(--lime);
  font: 700 11px "DM Sans", sans-serif;
  text-align: center;
}

.practice-list {
  display: grid;
  gap: 12px;
}

.practice-item {
  padding: 14px;
  border-radius: 10px;
  background: rgba(0, 0, 0, 0.1);
}

.practice-item > p {
  margin-bottom: 10px;
  color: rgba(255, 255, 255, 0.76);
  font-size: 12px;
  line-height: 1.65;
}

.practice-item > p span {
  display: inline-flex;
  margin-right: 8px;
  color: var(--lime);
  font-weight: 700;
}

.practice-options {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 8px;
}

.practice-option {
  min-height: 42px;
  padding: 9px 10px;
  border: 1px solid rgba(255, 255, 255, 0.12);
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.035);
  color: rgba(255, 255, 255, 0.78);
  font-size: 10px;
  text-align: left;
  cursor: pointer;
  transition: 0.2s ease;
}

.practice-option:hover:not(:disabled),
.practice-option.selected {
  border-color: var(--lime);
  background: rgba(200, 241, 53, 0.08);
  color: white;
}

.practice-option.correct {
  border-color: #79d4ac;
  background: rgba(77, 183, 135, 0.15);
  color: white;
}

.practice-option.wrong {
  border-color: #ff8a74;
  background: rgba(255, 95, 75, 0.12);
  color: white;
}

.practice-feedback {
  margin-top: 9px;
  padding-left: 11px;
  border-left: 2px solid var(--lime);
  color: rgba(255, 255, 255, 0.62);
  font-size: 10px;
  line-height: 1.65;
}

.lesson-actions {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-top: 27px;
}

.button-complete {
  min-height: 44px;
  background: var(--lime);
  color: var(--navy);
}

.button-complete.completed {
  background: rgba(255, 255, 255, 0.12);
  color: rgba(255, 255, 255, 0.7);
}

.text-button {
  border: 0;
  background: transparent;
  color: rgba(255, 255, 255, 0.7);
  font-size: 11px;
  font-weight: 700;
  cursor: pointer;
}

.text-button span {
  margin-left: 8px;
  color: var(--lime);
}

.lab-section {
  background: #edf1ea;
}

.lab-card {
  display: grid;
  grid-template-columns: 1.25fr 0.75fr;
  min-height: 500px;
  border-radius: 22px;
  background: var(--white);
  box-shadow: var(--shadow);
  overflow: hidden;
}

.lab-stage {
  position: relative;
  min-height: 500px;
  background: var(--navy);
  overflow: hidden;
}

#fieldCanvas {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
}

.lab-charge {
  position: absolute;
  top: 50%;
  z-index: 2;
  width: 82px;
  height: 82px;
  display: grid;
  place-items: center;
  border-radius: 50%;
  color: white;
  transform: translate(-50%, -50%);
  box-shadow: 0 16px 30px rgba(0, 0, 0, 0.28);
  transition: left 0.25s ease, background 0.25s ease;
}

.lab-charge-a { left: 30%; }
.lab-charge-b { left: 70%; }

.lab-charge span {
  font: 500 32px "DM Sans", sans-serif;
}

.lab-charge small {
  position: absolute;
  bottom: -30px;
  color: rgba(255, 255, 255, 0.6);
  font: 600 10px "DM Sans", sans-serif;
}

.distance-line {
  position: absolute;
  z-index: 1;
  top: calc(50% + 70px);
  left: 30%;
  right: 30%;
  height: 1px;
  border-top: 1px dashed rgba(255, 255, 255, 0.35);
  text-align: center;
  transition: left 0.25s ease, right 0.25s ease;
}

.distance-line::before,
.distance-line::after {
  content: "";
  position: absolute;
  top: -4px;
  width: 1px;
  height: 8px;
  background: rgba(255, 255, 255, 0.5);
}

.distance-line::before { left: 0; }
.distance-line::after { right: 0; }

.distance-line span {
  position: relative;
  top: 6px;
  padding: 2px 8px;
  border-radius: 4px;
  background: var(--navy);
  color: rgba(255, 255, 255, 0.6);
  font: 600 10px "DM Sans", sans-serif;
}

.lab-controls {
  padding: 40px;
}

.result-block {
  display: grid;
  grid-template-columns: 1fr auto;
  margin-bottom: 40px;
  padding-bottom: 26px;
  border-bottom: 1px solid var(--line);
}

.result-block small {
  grid-column: 1 / -1;
  color: #74867f;
  font-size: 11px;
  font-weight: 600;
}

.result-block strong {
  margin-top: 5px;
  font: 700 42px "DM Sans", sans-serif;
  letter-spacing: -0.04em;
}

.result-block span {
  align-self: center;
  padding: 5px 10px;
  border-radius: 999px;
  background: var(--mint);
  color: #227158;
  font-size: 10px;
  font-weight: 700;
}

.control-row {
  display: block;
  margin-bottom: 28px;
}

.control-row > span {
  display: flex;
  justify-content: space-between;
  margin-bottom: 12px;
}

.control-row strong,
.control-row output {
  font-size: 11px;
}

.control-row output {
  color: var(--ink-soft);
  font-family: "DM Sans", sans-serif;
}

input[type="range"] {
  width: 100%;
  height: 4px;
  border-radius: 99px;
  accent-color: var(--lime-dark);
  cursor: pointer;
}

.lab-insight {
  padding: 14px 16px;
  border-radius: 8px;
  background: #f1f5ed;
  color: #64776f;
  font-size: 10px;
}

.lab-insight strong {
  color: var(--ink);
}

.formula-section {
  background: var(--white);
}

.formula-deck {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 14px;
}

.formula-card {
  min-height: 205px;
  padding: 24px;
  border: 1px solid var(--line);
  border-radius: 14px;
  background: #fafbf8;
  text-align: left;
  cursor: pointer;
  transition: 0.25s ease;
}

.formula-card:hover,
.formula-card.active {
  border-color: var(--navy);
  background: var(--navy);
  color: white;
  transform: translateY(-4px);
  box-shadow: 0 16px 32px rgba(12, 37, 40, 0.15);
}

.formula-card > span {
  color: #92a198;
  font: 700 9px "DM Sans", sans-serif;
  letter-spacing: 0.15em;
}

.formula-card h3 {
  margin: 20px 0 12px;
  font: 600 23px "DM Sans", sans-serif;
}

.formula-card p {
  margin: 0;
  color: #71827a;
  font-size: 10px;
  line-height: 1.7;
}

.formula-card:hover p,
.formula-card.active p {
  color: rgba(255, 255, 255, 0.62);
}

.formula-card .formula-detail {
  display: none;
  margin-top: 13px;
  color: var(--lime);
}

.formula-card.active .formula-detail {
  display: block;
}

.quiz-section {
  padding-top: 70px;
  padding-bottom: 120px;
  background: var(--white);
}

.quiz-shell {
  display: grid;
  grid-template-columns: 0.38fr 0.62fr;
  min-height: 540px;
  border-radius: 24px;
  background: var(--navy);
  color: white;
  box-shadow: 0 28px 70px rgba(12, 37, 40, 0.18);
  overflow: hidden;
}

.quiz-intro {
  position: relative;
  padding: 55px 46px;
  background:
    radial-gradient(circle at 0 100%, rgba(200, 241, 53, 0.12), transparent 40%),
    var(--navy-light);
  overflow: hidden;
}

.quiz-intro::after {
  content: "?";
  position: absolute;
  right: -20px;
  bottom: -150px;
  color: rgba(255, 255, 255, 0.025);
  font: 700 430px/1 "DM Sans", sans-serif;
}

.quiz-intro .eyebrow {
  color: var(--lime);
}

.quiz-intro > p:not(.eyebrow) {
  margin-top: 20px;
  color: rgba(255, 255, 255, 0.55);
  font-size: 12px;
}

.quiz-score-ring {
  position: relative;
  z-index: 1;
  width: 128px;
  height: 128px;
  display: grid;
  place-items: center;
  margin-top: 44px;
  border: 1px solid rgba(200, 241, 53, 0.28);
  border-radius: 50%;
  background: rgba(200, 241, 53, 0.06);
}

.quiz-score-ring strong {
  font: 700 38px "DM Sans", sans-serif;
}

.quiz-score-ring span {
  color: rgba(255, 255, 255, 0.5);
  font: 600 12px "DM Sans", sans-serif;
}

.quiz-panel {
  padding: 55px 60px;
}

.quiz-progress {
  display: flex;
  align-items: center;
  gap: 20px;
}

.quiz-progress > span {
  flex: 0 0 auto;
  color: var(--lime);
  font: 700 9px "DM Sans", sans-serif;
  letter-spacing: 0.16em;
}

.quiz-progress > div {
  width: 100%;
  height: 2px;
  background: rgba(255, 255, 255, 0.1);
}

.quiz-progress i {
  display: block;
  width: 20%;
  height: 100%;
  background: var(--lime);
  transition: width 0.3s ease;
}

.quiz-panel h3 {
  min-height: 80px;
  margin: 40px 0 30px;
  font-size: 19px;
  line-height: 1.7;
}

.answer-list {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 10px;
}

.answer-button {
  min-height: 58px;
  padding: 12px 16px;
  border: 1px solid rgba(255, 255, 255, 0.13);
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.035);
  color: rgba(255, 255, 255, 0.8);
  font-size: 12px;
  text-align: left;
  cursor: pointer;
  transition: 0.2s ease;
}

.answer-button:hover:not(:disabled),
.answer-button.selected {
  border-color: var(--lime);
  background: rgba(200, 241, 53, 0.09);
  color: white;
}

.answer-button.correct {
  border-color: #79d4ac;
  background: rgba(77, 183, 135, 0.15);
}

.answer-button.wrong {
  border-color: #ff8a74;
  background: rgba(255, 95, 75, 0.12);
}

.answer-feedback {
  margin-top: 14px;
  padding: 12px 14px;
  border-left: 2px solid var(--lime);
  background: rgba(255, 255, 255, 0.04);
  color: rgba(255, 255, 255, 0.66);
  font-size: 10px;
}

.quiz-next {
  min-width: 150px;
  margin-top: 22px;
  background: var(--lime);
  color: var(--navy);
  box-shadow: none;
}

.quiz-next:disabled {
  opacity: 0.35;
  cursor: not-allowed;
}

footer {
  min-height: 130px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 30px max(24px, calc((100% - 1180px) / 2));
  background: #071a1c;
  color: white;
}

.footer-brand .brand-mark {
  background: var(--lime);
  color: var(--navy);
}

.footer-brand small,
footer > p {
  color: rgba(255, 255, 255, 0.4);
}

footer > p {
  margin: 0;
  font-size: 10px;
}

@media (max-width: 980px) {
  .hero {
    grid-template-columns: 1fr;
  }

  .hero-copy {
    max-width: 700px;
  }

  .hero-visual {
    min-height: 450px;
  }

  .learning-layout,
  .lab-card {
    grid-template-columns: 1fr;
  }

  .chapter-list {
    display: grid;
    grid-template-columns: 1fr 1fr;
  }

  .formula-deck {
    grid-template-columns: 1fr 1fr;
  }

  .quiz-shell {
    grid-template-columns: 1fr;
  }

  .quiz-intro {
    min-height: 360px;
  }
}

@media (max-width: 680px) {
  .site-header {
    width: calc(100% - 32px);
  }

  .main-nav {
    display: none;
  }

  .progress-pill {
    padding: 9px 12px;
  }

  .hero {
    width: calc(100% - 32px);
    min-height: auto;
    padding: 55px 0 70px;
    gap: 45px;
  }

  h1 {
    font-size: 42px;
  }

  .hero-actions {
    align-items: stretch;
    flex-direction: column;
  }

  .hero-stats {
    gap: 25px;
  }

  .hero-visual {
    min-height: 360px;
  }

  .charge {
    width: 78px;
    height: 78px;
  }

  .charge-positive { left: 14%; }
  .charge-negative { right: 14%; }

  .visual-caption strong {
    font-size: 14px;
  }

  .section {
    padding: 78px 16px;
  }

  .section-heading {
    align-items: flex-start;
    flex-direction: column;
    gap: 15px;
    margin-bottom: 34px;
  }

  .chapter-list {
    display: flex;
  }

  .lesson-card {
    padding: 30px 22px;
  }

  .concept-grid,
  .formula-panel,
  .practice-options {
    grid-template-columns: 1fr;
  }

  .lesson-actions {
    align-items: stretch;
    flex-direction: column;
    gap: 18px;
  }

  .lab-stage {
    min-height: 380px;
  }

  .lab-controls {
    padding: 30px 24px;
  }

  .lab-charge {
    width: 64px;
    height: 64px;
  }

  .formula-deck,
  .answer-list {
    grid-template-columns: 1fr;
  }

  .quiz-intro,
  .quiz-panel {
    padding: 38px 24px;
  }

  .quiz-panel h3 {
    margin-top: 30px;
  }

  footer {
    align-items: flex-start;
    flex-direction: column;
    gap: 24px;
  }
}

@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    scroll-behavior: auto !important;
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}