:root {
  --color-primary: #243e51;
  --color-primary-hover: #1b2f3d;
  --color-primary-accent: #36586d;
  --color-primary-soft: #e7edf2;
  --color-tertiary: #f1ebce;
  --green: #43a047;
  --muted: #546e7a;
  --muted-soft: #7a8c95;
  --text: #21303c;
  --page-bg: #f5f5f5;
  --border-soft: rgba(36, 62, 81, 0.12);
  --border-hairline: rgba(36, 62, 81, 0.08);
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  min-height: 100vh;
  background: var(--page-bg);
  color: var(--text);
  line-height: 1.6;
  font-size: 1.0625rem;
}

img {
  display: block;
  max-width: 100%;
}

a {
  color: var(--color-primary-accent);
  text-decoration-thickness: 1px;
  text-underline-offset: 0.18em;
}

a:hover {
  color: var(--color-primary);
}

.page-bg {
  position: fixed;
  inset: 0;
  z-index: -1;
  pointer-events: none;
  background:
    radial-gradient(ellipse 72% 56% at 12% -10%, rgba(36, 62, 81, 0.07), transparent 52%),
    radial-gradient(ellipse 58% 44% at 100% 20%, rgba(67, 160, 71, 0.06), transparent 46%),
    var(--page-bg);
}

.wrap {
  width: min(72rem, calc(100% - 2rem));
  margin-inline: auto;
}

.wrap.narrow {
  width: min(42rem, calc(100% - 2rem));
}

/* ---- Header ---- */
.site-header {
  position: sticky;
  top: 0;
  z-index: 1000;
  border-bottom: 1px solid var(--border-hairline);
  background: rgba(245, 245, 245, 0.84);
  backdrop-filter: blur(14px);
  -webkit-backdrop-filter: blur(14px);
}

.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  padding: 0.9rem 0;
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 0.7rem;
  color: var(--color-primary);
  font-weight: 800;
  font-size: 1.15rem;
  letter-spacing: -0.02em;
  text-decoration: none;
}

.brand-mark {
  width: 2.4rem;
  height: 2.4rem;
  border-radius: 0.8rem;
  display: grid;
  place-items: center;
  background: var(--color-tertiary);
  border: 1px solid rgba(36, 62, 81, 0.12);
}

.brand-mark img {
  width: 1.45rem;
  height: 1.45rem;
}

.site-nav {
  display: inline-flex;
  align-items: center;
  gap: 0.2rem;
  flex-wrap: wrap;
  justify-content: center;
}

.site-nav-link {
  display: inline-flex;
  align-items: center;
  padding: 0.45rem 0.75rem;
  border-radius: 999px;
  color: var(--color-primary);
  font-size: 0.92rem;
  font-weight: 700;
  text-decoration: none;
}

.site-nav-link:hover,
.site-nav-link.is-active {
  background: rgba(36, 62, 81, 0.08);
}

.header-cta,
.button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  padding: 0.7rem 1.15rem;
  border-radius: 999px;
  border: 1px solid rgba(36, 62, 81, 0.14);
  background: #fff;
  color: var(--color-primary);
  font-size: 0.94rem;
  font-weight: 700;
  text-decoration: none;
  transition: transform 0.18s ease, box-shadow 0.18s ease, border-color 0.18s ease;
}

.header-cta:hover,
.button:hover {
  transform: translateY(-1px);
  box-shadow: 0 8px 20px rgba(36, 62, 81, 0.1);
  border-color: rgba(36, 62, 81, 0.26);
  color: var(--color-primary);
}

.button.primary {
  background: var(--color-primary);
  color: #fff;
  border-color: var(--color-primary);
}

.button.primary:hover {
  background: var(--color-primary-hover);
  color: #fff;
}

.site-main {
  padding-bottom: 2rem;
}

/* ---- Breadcrumbs ---- */
.breadcrumbs ol {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
  padding: 0;
  margin: 0 0 2rem;
  list-style: none;
  color: var(--muted-soft);
  font-size: 0.85rem;
}

.breadcrumbs li {
  display: inline-flex;
  align-items: center;
}

.breadcrumbs li + li::before {
  content: '/';
  margin-right: 0.4rem;
  opacity: 0.5;
}

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

.breadcrumbs a:hover {
  color: var(--color-primary);
}

/* ---- Hub page ---- */
.hub-hero {
  padding: 3rem 0 1.25rem;
}

.hub-title {
  margin: 0 0 0.85rem;
  font-size: clamp(2.4rem, 5vw, 3.6rem);
  line-height: 1.05;
  letter-spacing: -0.03em;
  color: var(--color-primary);
  font-weight: 800;
}

.hub-lede {
  margin: 0;
  font-size: 1.2rem;
  color: var(--muted);
  max-width: 36ch;
}

/* ---- Detail page hero ---- */
.detail-hero {
  padding: 2.5rem 0 1rem;
}

.detail-hero .eyebrow {
  margin: 0 0 0.8rem;
  color: var(--color-primary-accent);
  font-size: 0.75rem;
  font-weight: 800;
  letter-spacing: 0.12em;
  text-transform: uppercase;
}

.detail-hero h1 {
  margin: 0 0 1rem;
  font-size: clamp(2rem, 4.2vw, 3rem);
  line-height: 1.08;
  letter-spacing: -0.025em;
  color: var(--color-primary);
  font-weight: 800;
}

.detail-hero .answer {
  margin: 0;
  font-size: 1.15rem;
  line-height: 1.55;
  color: var(--text);
  max-width: 44ch;
}

/* ---- Detail prose ---- */
.detail-prose {
  padding: 1.5rem 0 2rem;
}

.detail-prose h2 {
  margin: 1.6rem 0 0.6rem;
  font-size: 1.2rem;
  line-height: 1.3;
  letter-spacing: -0.015em;
  color: var(--color-primary);
  font-weight: 700;
}

.detail-prose h2:first-child {
  margin-top: 0;
}

.detail-prose p {
  margin: 0 0 1rem;
  color: var(--text);
}

.detail-prose p:last-child {
  margin-bottom: 0;
}

.detail-prose ul {
  margin: 0.5rem 0 1rem;
  padding-left: 1.15rem;
  color: var(--text);
}

.detail-prose li + li {
  margin-top: 0.4rem;
}

/* ---- Article teaching blocks ---- */
.article-callout,
.formula-card,
.svg-visual,
.article-calculator {
  margin: 1.4rem 0;
  border: 1px solid var(--border-soft);
  border-radius: 1rem;
  background: rgba(255, 255, 255, 0.78);
  box-shadow: 0 16px 40px rgba(36, 62, 81, 0.06);
}

.article-callout {
  padding: 1rem;
  border-left: 0.35rem solid var(--green);
}

.article-callout.technical {
  border-left-color: var(--color-primary-accent);
  background: rgba(231, 237, 242, 0.72);
}

.article-callout-label,
.formula-label,
.scenario-kicker {
  margin: 0 0 0.35rem;
  color: var(--muted-soft);
  font-size: 0.72rem;
  font-weight: 800;
  letter-spacing: 0.1em;
  text-transform: uppercase;
}

.article-callout h3,
.calculator-heading h3,
.scenario-card h3 {
  margin: 0 0 0.45rem;
  color: var(--color-primary);
  font-size: 1rem;
  line-height: 1.3;
  letter-spacing: -0.01em;
}

.article-callout p,
.article-calculator p,
.scenario-card p,
.formula-card p,
.svg-visual p {
  color: var(--muted);
  font-size: 0.94rem;
}

.article-callout p:last-child,
.formula-card p:last-child,
.svg-visual p:last-child,
.article-calculator p:last-child,
.scenario-card p:last-child {
  margin-bottom: 0;
}

.formula-card {
  padding: 1rem;
}

.formula-card figcaption,
.svg-visual figcaption {
  margin: 0 0 0.65rem;
  color: var(--color-primary);
  font-weight: 800;
  letter-spacing: -0.01em;
}

.formula-expression {
  display: block;
  overflow-x: auto;
  max-width: 100%;
  margin: 0;
  padding: 0.9rem;
  border: 1px solid rgba(36, 62, 81, 0.16);
  border-radius: 0.75rem;
  background: #fff;
  color: var(--color-primary);
  font-family: Georgia, 'Times New Roman', serif;
  font-size: clamp(1rem, 3vw, 1.18rem);
  font-weight: 700;
  line-height: 1.5;
  white-space: nowrap;
}

.formula-expression + p {
  margin-top: 0.75rem;
}

.svg-visual {
  padding: 1rem;
}

.svg-visual-frame {
  overflow: hidden;
  border-radius: 0.85rem;
  background: #fff;
}

.svg-visual svg {
  display: block;
  width: 100%;
  height: auto;
}

.svg-visual figcaption + .svg-visual-frame {
  margin-top: 0.2rem;
}

.svg-visual-frame + p {
  margin-top: 0.75rem;
}

.scenario-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 0.9rem;
  margin: 1.4rem 0;
}

.scenario-card {
  min-width: 0;
  padding: 1rem;
  border: 1px solid var(--border-soft);
  border-radius: 1rem;
  background: rgba(255, 255, 255, 0.8);
}

.scenario-metric {
  margin: 0 0 0.8rem;
  color: var(--color-primary);
  font-weight: 800;
}

.scenario-sparkline {
  display: block;
  width: 100%;
  height: auto;
  min-height: 4.6rem;
  margin: 0.25rem 0 0.8rem;
  color: var(--green);
}

.scenario-card small {
  display: block;
  margin-top: 0.65rem;
  color: var(--muted-soft);
  font-size: 0.82rem;
  line-height: 1.45;
}

.article-calculator {
  padding: 1rem;
}

.calculator-heading {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 1rem;
  margin-bottom: 1rem;
}

.calculator-heading p:last-child {
  margin-bottom: 0;
}

.unit-toggle {
  display: inline-flex;
  flex: 0 0 auto;
  align-items: center;
  gap: 0.25rem;
  margin: 0;
  padding: 0.25rem;
  border: 1px solid var(--border-soft);
  border-radius: 999px;
  background: #fff;
}

.unit-toggle legend {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
}

.unit-toggle label {
  cursor: pointer;
}

.unit-toggle input {
  position: absolute;
  opacity: 0;
  pointer-events: none;
}

.unit-toggle span {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-width: 2.3rem;
  min-height: 2rem;
  border-radius: 999px;
  color: var(--muted);
  font-size: 0.82rem;
  font-weight: 800;
}

.unit-toggle input:checked + span {
  background: var(--color-primary);
  color: #fff;
}

.unit-toggle input:focus-visible + span {
  outline: 3px solid rgba(67, 160, 71, 0.22);
  outline-offset: 2px;
}

.calculator-controls {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 0.8rem;
}

.calculator-controls label {
  display: grid;
  gap: 0.35rem;
  color: var(--color-primary);
  font-size: 0.84rem;
  font-weight: 700;
}

.calculator-controls input {
  width: 100%;
  min-height: 2.75rem;
  border: 1px solid var(--border-soft);
  border-radius: 0.75rem;
  padding: 0.55rem 0.7rem;
  color: var(--text);
  font: inherit;
  background: #fff;
}

.calculator-controls input:focus {
  outline: 3px solid rgba(67, 160, 71, 0.18);
  border-color: var(--green);
}

.calculator-results {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 0.75rem;
  margin-top: 1rem;
}

.calculator-results p {
  display: grid;
  gap: 0.15rem;
  margin: 0;
  padding: 0.8rem;
  border-radius: 0.8rem;
  background: var(--color-primary-soft);
}

.calculator-results span {
  color: var(--muted);
  font-size: 0.78rem;
  font-weight: 700;
}

.calculator-results strong {
  color: var(--color-primary);
  font-size: 1.25rem;
  line-height: 1.2;
}

/* ---- Link list (shared by hub + related) ---- */
.link-list-section {
  padding: 1rem 0 2rem;
}

.minor-heading {
  margin: 0 0 0.75rem;
  font-size: 0.78rem;
  font-weight: 800;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--muted-soft);
}

.link-list {
  list-style: none;
  padding: 0;
  margin: 0;
  border-top: 1px solid var(--border-hairline);
}

.link-list li {
  border-bottom: 1px solid var(--border-hairline);
}

.link-row {
  display: flex;
  align-items: center;
  gap: 1.25rem;
  padding: 1.1rem 0.25rem;
  color: var(--text);
  text-decoration: none;
  transition: padding 0.18s ease;
}

.link-row:hover {
  padding-inline: 0.6rem;
}

.link-row-main {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
  min-width: 0;
}

.link-row-title {
  font-size: 1.1rem;
  font-weight: 700;
  color: var(--color-primary);
  letter-spacing: -0.012em;
}

.link-row-summary {
  color: var(--muted);
  font-size: 0.95rem;
  line-height: 1.5;
}

.link-row-arrow {
  font-size: 1.15rem;
  color: var(--color-primary-accent);
  opacity: 0.4;
  transition: transform 0.18s ease, opacity 0.18s ease;
}

.link-row:hover .link-row-arrow {
  opacity: 1;
  transform: translateX(4px);
}

@media (max-width: 560px) {
  .link-row {
    padding: 1rem 0.25rem;
    gap: 0.85rem;
  }

  .link-row-arrow {
    display: none;
  }
}

/* ---- Optional beginner note ---- */
.beginner-note-section {
  padding: 0.5rem 0 1.25rem;
}

.beginner-note {
  border: 1px dashed rgba(36, 62, 81, 0.22);
  background: rgba(255, 255, 255, 0.72);
  border-radius: 0.75rem;
  padding: 0.95rem 1rem;
}

.beginner-note-label {
  margin: 0 0 0.25rem;
  color: var(--muted-soft);
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.beginner-note h2 {
  margin: 0 0 0.35rem;
  color: var(--color-primary);
  font-size: 1rem;
  line-height: 1.35;
  letter-spacing: -0.01em;
}

.beginner-note p {
  margin: 0;
  color: var(--muted);
  font-size: 0.92rem;
}

.beginner-note ul {
  margin: 0.5rem 0 0;
  padding-left: 1rem;
  color: var(--muted);
  font-size: 0.9rem;
}

.beginner-note li + li {
  margin-top: 0.3rem;
}

/* ---- FAQ ---- */
.detail-faq {
  padding: 1rem 0 2rem;
}

.faq-list {
  border-top: 1px solid var(--border-hairline);
}

.faq-item {
  border-bottom: 1px solid var(--border-hairline);
}

.faq-item summary {
  cursor: pointer;
  list-style: none;
  padding: 1rem 0.25rem;
  color: var(--color-primary);
  font-weight: 600;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.faq-item summary::-webkit-details-marker {
  display: none;
}

.faq-item summary::after {
  content: '+';
  font-size: 1.3rem;
  font-weight: 400;
  color: var(--muted-soft);
  transition: transform 0.2s ease;
}

.faq-item[open] summary::after {
  transform: rotate(45deg);
}

.faq-item p {
  padding: 0 0.25rem 1.1rem;
  margin: 0;
  color: var(--muted);
}

/* ---- Final CTA ---- */
.final-cta {
  padding: 2rem 0 3rem;
  text-align: center;
  border-top: 1px solid var(--border-hairline);
  margin-top: 1rem;
}

.final-cta h2 {
  margin: 0 0 1.25rem;
  font-size: clamp(1.3rem, 2.4vw, 1.7rem);
  line-height: 1.2;
  letter-spacing: -0.02em;
  color: var(--color-primary);
  font-weight: 700;
}

/* ---- Footer ---- */
.site-footer {
  border-top: 1px solid var(--border-hairline);
  padding: 1.2rem 0 2rem;
}

.footer-inner {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: 0.8rem;
  color: var(--muted);
  font-size: 0.88rem;
}

.footer-inner p {
  margin: 0;
}

.footer-links {
  display: inline-flex;
  align-items: center;
  gap: 0.8rem;
  flex-wrap: wrap;
}

.footer-inner a {
  color: var(--muted);
}

/* ---- Responsive ---- */
@media (max-width: 799px) {
  .header-inner {
    flex-wrap: wrap;
    justify-content: center;
  }

  .site-nav {
    order: 3;
    width: 100%;
  }
}

@media (max-width: 640px) {
  .hub-hero {
    padding-top: 2rem;
  }

  .detail-hero {
    padding-top: 1.5rem;
  }

  .calculator-heading {
    align-items: stretch;
    flex-direction: column;
  }

  .unit-toggle {
    align-self: flex-start;
  }

  .scenario-grid,
  .calculator-controls,
  .calculator-results {
    grid-template-columns: 1fr;
  }

  .article-callout,
  .formula-card,
  .svg-visual,
  .article-calculator,
  .scenario-card {
    border-radius: 0.85rem;
  }

  .formula-expression {
    white-space: normal;
    overflow-wrap: anywhere;
  }
}