:root {
  --font-scale: 1;
  --bg: #f6f8fb;
  --surface: #ffffff;
  --surface-soft: #edf7f5;
  --text: #172026;
  --muted: #5f6f76;
  --border: #d7e1e4;
  --accent: #0f766e;
  --accent-strong: #0b5f59;
  --accent-soft: #d8f3ee;
  --blue: #2563eb;
  --blue-soft: #dbeafe;
  --amber: #b45309;
  --amber-soft: #ffedd5;
  --red: #b91c1c;
  --red-soft: #fee2e2;
  --green: #15803d;
  --green-soft: #dcfce7;
  --code-bg: #111827;
  --code-text: #e5e7eb;
  --shadow: 0 18px 48px rgba(23, 32, 38, 0.1);
}

[data-theme="dark"] {
  --bg: #101416;
  --surface: #182022;
  --surface-soft: #162b2b;
  --text: #eef6f4;
  --muted: #b5c6c2;
  --border: #314044;
  --accent: #5eead4;
  --accent-strong: #99f6e4;
  --accent-soft: #123b39;
  --blue: #93c5fd;
  --blue-soft: #142844;
  --amber: #fbbf24;
  --amber-soft: #3b2b13;
  --red: #fca5a5;
  --red-soft: #3a1818;
  --green: #86efac;
  --green-soft: #15351f;
  --code-bg: #05070a;
  --code-text: #f8fafc;
  --shadow: 0 18px 48px rgba(0, 0, 0, 0.35);
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: Arial, Helvetica, sans-serif;
  font-size: calc(16px * var(--font-scale));
  line-height: 1.65;
}

a {
  color: var(--accent-strong);
}

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

button {
  font: inherit;
}

.skip-link {
  position: absolute;
  left: 1rem;
  top: -4rem;
  z-index: 20;
  background: var(--surface);
  border: 2px solid var(--accent);
  border-radius: 6px;
  color: var(--text);
  padding: 0.65rem 0.85rem;
}

.skip-link:focus {
  top: 1rem;
}

.topbar {
  position: sticky;
  top: 0;
  z-index: 10;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  padding: 0.85rem clamp(1rem, 4vw, 2.5rem);
  background: color-mix(in srgb, var(--surface) 92%, transparent);
  border-bottom: 1px solid var(--border);
  backdrop-filter: blur(16px);
}

.brand-block {
  display: flex;
  align-items: center;
  gap: 0.8rem;
  min-width: 0;
}

.brand-mark {
  display: grid;
  place-items: center;
  flex: 0 0 auto;
  width: 3rem;
  height: 3rem;
  border-radius: 8px;
  background: var(--accent);
  color: #ffffff;
  font-weight: 800;
  letter-spacing: 0;
}

.eyebrow {
  margin: 0 0 0.2rem;
  color: var(--accent-strong);
  font-size: 0.78em;
  font-weight: 800;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.brand-block h1 {
  margin: 0;
  color: var(--text);
  font-size: 1.02rem;
  line-height: 1.2;
}

.toolbar {
  display: flex;
  align-items: center;
  gap: 0.45rem;
}

.icon-button,
.theme-toggle,
.primary-button,
.secondary-button,
.quiz-tab,
.primary-link,
.secondary-link {
  min-height: 2.55rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  cursor: pointer;
  transition: transform 160ms ease, border-color 160ms ease, background 160ms ease;
}

.icon-button:hover,
.theme-toggle:hover,
.primary-button:hover,
.secondary-button:hover,
.quiz-tab:hover,
.primary-link:hover,
.secondary-link:hover {
  transform: translateY(-1px);
  border-color: var(--accent);
}

.icon-button {
  width: 2.7rem;
  background: var(--surface);
  color: var(--text);
  font-weight: 800;
}

.theme-toggle {
  display: inline-flex;
  align-items: center;
  gap: 0.45rem;
  background: var(--surface);
  color: var(--text);
  padding: 0 0.8rem;
}

.theme-icon {
  width: 1rem;
  height: 1rem;
  border-radius: 999px;
  background: var(--amber);
  box-shadow: inset -0.28rem -0.28rem 0 var(--surface);
}

.unit-nav {
  position: sticky;
  top: 4.75rem;
  z-index: 9;
  display: flex;
  gap: 0.55rem;
  overflow-x: auto;
  padding: 0.75rem clamp(1rem, 4vw, 2.5rem);
  background: var(--bg);
  border-bottom: 1px solid var(--border);
}

.unit-nav a {
  flex: 0 0 auto;
  border: 1px solid var(--border);
  border-radius: 999px;
  background: var(--surface);
  color: var(--text);
  padding: 0.45rem 0.8rem;
  text-decoration: none;
  font-size: 0.92em;
  font-weight: 700;
}

main {
  max-width: 1180px;
  margin: 0 auto;
  padding: clamp(1rem, 4vw, 2.5rem);
}

.hero-section,
.quick-revision,
.unit-section,
.quiz-section {
  margin-bottom: clamp(2rem, 7vw, 4rem);
}

.hero-section {
  display: grid;
  grid-template-columns: minmax(0, 1.08fr) minmax(280px, 0.92fr);
  gap: clamp(1.2rem, 4vw, 3rem);
  align-items: center;
  min-height: calc(100vh - 10rem);
  padding: clamp(1rem, 4vw, 2.2rem) 0;
}

.hero-copy h2 {
  margin: 0;
  max-width: 12ch;
  font-size: clamp(2.25rem, 7vw, 5.4rem);
  line-height: 0.98;
  letter-spacing: 0;
}

.hero-copy p:not(.eyebrow) {
  max-width: 68ch;
  color: var(--muted);
  font-size: 1.05em;
}

.hero-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  margin-top: 1.35rem;
}

.primary-link,
.secondary-link {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  padding: 0.75rem 1rem;
  text-decoration: none;
  font-weight: 800;
}

.primary-link,
.primary-button {
  border-color: var(--accent);
  background: var(--accent);
  color: #ffffff;
}

.secondary-link,
.secondary-button {
  background: var(--surface);
  color: var(--text);
}

.iot-map {
  display: grid;
  gap: 0.75rem;
  padding: 1rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface);
  box-shadow: var(--shadow);
}

.map-node {
  border: 1px solid var(--border);
  border-left: 0.4rem solid var(--accent);
  border-radius: 8px;
  background: var(--surface-soft);
  padding: 1rem;
  font-weight: 800;
}

.map-node.sensor {
  border-left-color: var(--blue);
}

.map-node.network {
  border-left-color: var(--amber);
}

.map-node.cloud {
  border-left-color: var(--green);
}

.map-node.app {
  border-left-color: var(--red);
}

.map-arrow {
  width: 0.14rem;
  height: 1.6rem;
  margin: 0 auto;
  background: var(--border);
  position: relative;
}

.map-arrow::after {
  content: "";
  position: absolute;
  left: 50%;
  bottom: -0.15rem;
  width: 0.65rem;
  height: 0.65rem;
  border-right: 0.16rem solid var(--border);
  border-bottom: 0.16rem solid var(--border);
  transform: translateX(-50%) rotate(45deg);
}

.quick-revision {
  display: grid;
  gap: 1rem;
}

.quick-revision h2,
.unit-heading h2 {
  margin: 0;
  font-size: clamp(1.8rem, 4vw, 3.2rem);
  line-height: 1.08;
}

.revision-grid,
.application-grid {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 0.8rem;
}

.revision-grid article,
.application-grid div,
.decision-grid div,
.dashboard-wireframe div {
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface);
  padding: 1rem;
}

.revision-grid h3,
.note-panel h3 {
  margin: 0 0 0.7rem;
  font-size: 1.22em;
  line-height: 1.25;
}

.revision-grid p,
.unit-heading p,
.note-panel p {
  color: var(--muted);
}

.unit-section,
.quiz-section {
  scroll-margin-top: 8.5rem;
}

.unit-heading {
  margin-bottom: 1rem;
}

.unit-heading p:last-child {
  max-width: 78ch;
}

.two-column {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 1rem;
}

.note-panel {
  margin-bottom: 1rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface);
  padding: clamp(1rem, 3vw, 1.35rem);
}

.clean-list,
.component-list,
.step-list {
  margin: 0;
  padding-left: 1.2rem;
}

.clean-list li,
.component-list li,
.step-list li {
  margin: 0.45rem 0;
}

.chip-list {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
}

.chip-list span {
  border: 1px solid var(--border);
  border-radius: 999px;
  background: var(--accent-soft);
  color: var(--text);
  padding: 0.4rem 0.7rem;
  font-weight: 700;
}

.responsive-table {
  width: 100%;
  overflow-x: auto;
}

table {
  width: 100%;
  border-collapse: collapse;
  min-width: 620px;
}

th,
td {
  border: 1px solid var(--border);
  padding: 0.75rem;
  text-align: left;
  vertical-align: top;
}

th {
  background: var(--surface-soft);
  color: var(--text);
}

pre {
  overflow-x: auto;
  margin: 1rem 0 0;
  border-radius: 8px;
  background: var(--code-bg);
  color: var(--code-text);
  padding: 1rem;
}

code {
  font-family: Consolas, "Courier New", monospace;
  font-size: 0.9em;
}

.diagram-block,
.soc-diagram,
.flow-diagram,
.layer-stack,
.model-ladder,
.pin-board,
.node-red-flow,
.mqtt-diagram,
.dashboard-wireframe,
.decision-grid {
  margin: 1rem 0;
}

.embedded-block {
  display: grid;
  grid-template-columns: repeat(6, minmax(0, 1fr));
  gap: 0.5rem;
}

.embedded-block div,
.soc-diagram div,
.flow-card,
.node-red-flow div,
.mqtt-diagram div {
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface-soft);
  padding: 0.85rem;
  text-align: center;
  font-weight: 800;
}

.flow-diagram {
  display: flex;
  align-items: stretch;
  gap: 0.55rem;
}

.flow-diagram.compact .flow-card {
  font-size: 0.9em;
}

.flow-card {
  flex: 1 1 0;
  min-width: 8rem;
}

.flow-card span,
.mqtt-diagram span,
.application-grid span,
.decision-grid span,
.model-ladder span,
.layer-stack span {
  display: block;
  margin-top: 0.25rem;
  color: var(--muted);
  font-size: 0.86em;
  font-weight: 500;
}

.flow-line {
  flex: 0 0 1.25rem;
  align-self: center;
  height: 0.15rem;
  background: var(--border);
  position: relative;
}

.flow-line::after {
  content: "";
  position: absolute;
  right: -0.15rem;
  top: 50%;
  width: 0.52rem;
  height: 0.52rem;
  border-top: 0.15rem solid var(--border);
  border-right: 0.15rem solid var(--border);
  transform: translateY(-50%) rotate(45deg);
}

.layer-stack {
  display: grid;
  gap: 0.45rem;
}

.layer-stack div {
  border: 1px solid var(--border);
  border-left: 0.45rem solid var(--accent);
  border-radius: 8px;
  background: var(--surface-soft);
  padding: 0.85rem;
}

.layer-stack div:nth-child(2) {
  border-left-color: var(--blue);
}

.layer-stack div:nth-child(3) {
  border-left-color: var(--amber);
}

.layer-stack div:nth-child(4) {
  border-left-color: var(--green);
}

.layer-stack div:nth-child(5) {
  border-left-color: var(--red);
}

.exam-tip,
.formula-box {
  border-left: 0.35rem solid var(--amber);
  border-radius: 8px;
  background: var(--amber-soft);
  color: var(--text) !important;
  padding: 0.85rem 1rem;
}

.soc-diagram {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 0.55rem;
}

.mesh-diagram {
  display: grid;
  grid-template-columns: repeat(3, minmax(5rem, 1fr));
  gap: 0.75rem;
  max-width: 34rem;
  margin: 1rem auto;
  position: relative;
}

.mesh-diagram span {
  display: grid;
  place-items: center;
  min-height: 4.2rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--blue-soft);
  font-weight: 800;
}

.model-ladder {
  display: grid;
  gap: 0.6rem;
}

.model-ladder div {
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface-soft);
  padding: 0.85rem 1rem;
}

.bar-chart {
  display: grid;
  gap: 0.8rem;
}

.bar-chart div {
  display: grid;
  grid-template-columns: 8rem 1fr;
  align-items: center;
  gap: 0.7rem;
}

.bar-chart span {
  font-weight: 800;
}

.bar-chart b {
  display: block;
  height: 1rem;
  width: var(--bar);
  border-radius: 999px;
  background: linear-gradient(90deg, var(--accent), var(--blue));
}

.decision-grid,
.application-grid {
  grid-template-columns: repeat(2, minmax(0, 1fr));
}

.pin-board {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 0.35rem 0.6rem;
  max-width: 42rem;
}

.pin-board span {
  border: 1px solid var(--border);
  border-radius: 6px;
  background: var(--surface-soft);
  padding: 0.45rem 0.55rem;
  font-size: 0.88em;
  font-weight: 700;
}

.pin-board .power {
  background: var(--red-soft);
}

.pin-board .ground {
  background: var(--green-soft);
}

.formula-box {
  font-weight: 800;
}

.dashboard-wireframe {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 0.75rem;
}

.dashboard-wireframe .wide {
  grid-column: 1 / -1;
  min-height: 5rem;
  background: linear-gradient(135deg, var(--blue-soft), var(--accent-soft));
}

.node-red-flow,
.mqtt-diagram {
  display: grid;
  grid-template-columns: repeat(5, minmax(0, 1fr));
  gap: 0.6rem;
}

.mqtt-diagram {
  grid-template-columns: repeat(3, minmax(0, 1fr));
}

.quiz-shell {
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface);
  padding: 1rem;
}

.quiz-tabs {
  display: flex;
  gap: 0.55rem;
  overflow-x: auto;
  padding-bottom: 0.75rem;
}

.quiz-tab {
  flex: 0 0 auto;
  background: var(--surface-soft);
  color: var(--text);
  padding: 0.5rem 0.8rem;
  font-weight: 800;
}

.quiz-tab.active {
  background: var(--accent);
  color: #ffffff;
  border-color: var(--accent);
}

.quiz-card {
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--bg);
  padding: clamp(1rem, 3vw, 1.35rem);
}

.quiz-meta,
.score-strip,
.quiz-actions {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
  flex-wrap: wrap;
}

.quiz-meta,
.score-strip {
  color: var(--muted);
  font-weight: 800;
}

.quiz-card h3 {
  margin: 1rem 0;
  font-size: 1.35em;
  line-height: 1.3;
}

.quiz-options {
  display: grid;
  gap: 0.65rem;
}

.quiz-option {
  width: 100%;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface);
  color: var(--text);
  padding: 0.85rem;
  text-align: left;
  cursor: pointer;
}

.quiz-option.correct {
  border-color: var(--green);
  background: var(--green-soft);
}

.quiz-option.wrong {
  border-color: var(--red);
  background: var(--red-soft);
}

.quiz-option:disabled {
  cursor: default;
  opacity: 0.92;
}

.quiz-feedback {
  margin-top: 0.85rem;
  border-radius: 8px;
  padding: 0.85rem;
  font-weight: 800;
}

.quiz-feedback.good {
  background: var(--green-soft);
  color: var(--text);
}

.quiz-feedback.bad {
  background: var(--red-soft);
  color: var(--text);
}

.quiz-actions {
  margin-top: 1rem;
}

.primary-button,
.secondary-button {
  padding: 0.65rem 0.95rem;
  font-weight: 800;
}

.site-footer {
  display: flex;
  justify-content: space-between;
  gap: 1rem;
  flex-wrap: wrap;
  padding: 1.2rem clamp(1rem, 4vw, 2.5rem);
  border-top: 1px solid var(--border);
  background: var(--surface);
}

.credit {
  display: inline-flex;
  align-items: center;
  gap: 0.35rem;
  flex-wrap: wrap;
}

.heart {
  position: relative;
  display: inline-block;
  width: 1rem;
  height: 1rem;
  transform: rotate(-45deg);
  background: #e11d48;
  animation: pulse-heart 1.25s ease-in-out infinite;
}

.heart::before,
.heart::after {
  content: "";
  position: absolute;
  width: 1rem;
  height: 1rem;
  border-radius: 50%;
  background: #e11d48;
}

.heart::before {
  top: -0.5rem;
  left: 0;
}

.heart::after {
  top: 0;
  right: -0.5rem;
}

@keyframes pulse-heart {
  0%,
  100% {
    transform: rotate(-45deg) scale(0.92);
  }
  50% {
    transform: rotate(-45deg) scale(1.12);
  }
}

@media (max-width: 900px) {
  .hero-section,
  .two-column,
  .revision-grid,
  .application-grid,
  .decision-grid {
    grid-template-columns: 1fr;
  }

  .embedded-block,
  .soc-diagram,
  .node-red-flow {
    grid-template-columns: repeat(2, minmax(0, 1fr));
  }

  .flow-diagram {
    flex-direction: column;
  }

  .flow-line {
    width: 0.15rem;
    height: 1.5rem;
    align-self: center;
  }

  .flow-line::after {
    right: auto;
    left: 50%;
    top: auto;
    bottom: -0.15rem;
    transform: translateX(-50%) rotate(135deg);
  }
}

@media (max-width: 640px) {
  .topbar {
    align-items: flex-start;
    flex-direction: column;
  }

  .toolbar {
    width: 100%;
  }

  .theme-toggle {
    margin-left: auto;
  }

  .unit-nav {
    top: 7.6rem;
  }

  .hero-section {
    min-height: auto;
    padding-top: 1rem;
  }

  .hero-copy h2 {
    max-width: 11ch;
    font-size: 2.65rem;
  }

  .pin-board,
  .dashboard-wireframe,
  .mqtt-diagram {
    grid-template-columns: 1fr;
  }

  .bar-chart div {
    grid-template-columns: 1fr;
  }

  table {
    min-width: 540px;
  }
}

@media print {
  .topbar,
  .unit-nav,
  .quiz-section,
  .site-footer,
  .hero-actions {
    display: none;
  }

  body {
    background: #ffffff;
    color: #000000;
  }

  main {
    max-width: none;
    padding: 0;
  }

  .note-panel {
    break-inside: avoid;
    box-shadow: none;
  }
}