/* ═══════════════════════════════════════════════════════════════════════════
   UGP Physics — Interactive Explainer
   Global Design System
   ═══════════════════════════════════════════════════════════════════════════ */

/* ─── Design Tokens ──────────────────────────────────────────────────────── */
:root {
  /* Learning path accent colors */
  --path-1: #3B82F6;   /* blue     — Foundations */
  --path-2: #8B5CF6;   /* violet   — Selection */
  --path-3: #10B981;   /* emerald  — Particles */
  --path-4: #F59E0B;   /* amber    — Forces */
  --path-5: #6366F1;   /* indigo   — Spacetime */
  --path-6: #06B6D4;   /* cyan     — Quantum */
  --path-7: #F97316;   /* orange   — Cosmology */
  --path-8: #64748B;   /* slate    — Context */

  /* Set per-page: <body style="--path-current: var(--path-1)"> */
  --path-current: var(--path-1);
  --accent: var(--path-current);

  /* Surfaces */
  --bg-primary:   #111113;
  --bg-secondary: #18181B;
  --bg-elevated:  #1F1F23;
  --bg-card:      #27272A;

  /* Text */
  --text-primary:     #F4F4F5;
  --text-secondary:   #A1A1AA;
  --text-tertiary:    #71717A;

  /* Borders */
  --stroke-primary:   rgba(255, 255, 255, 0.12);
  --stroke-secondary: rgba(255, 255, 255, 0.07);

  /* Certification */
  --cert-lean: #22C55E;
  --cert-ad:   #3B82F6;
  --cert-b:    #F59E0B;
  --cert-conj: #EF4444;

  /* Radius */
  --r-sm: 4px;
  --r-md: 8px;
  --r-lg: 12px;
  --r-full: 9999px;
}

/* ─── Reset ──────────────────────────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }

body {
  font-family: 'Inter', system-ui, -apple-system, sans-serif;
  font-size: 16px;
  line-height: 1.75;
  color: var(--text-primary);
  background: var(--bg-primary);
  min-height: 100vh;
  -webkit-font-smoothing: antialiased;
}

a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

code {
  font-family: 'JetBrains Mono', 'Fira Code', monospace;
  font-size: 0.875em;
  background: rgba(255, 255, 255, 0.07);
  padding: 0.1em 0.38em;
  border-radius: var(--r-sm);
}

pre {
  font-family: 'JetBrains Mono', 'Fira Code', monospace;
  font-size: 0.875rem;
  background: var(--bg-elevated);
  border: 1px solid var(--stroke-primary);
  border-radius: var(--r-md);
  padding: 1rem;
  overflow-x: auto;
  margin: 1rem 0;
}

pre code { background: none; padding: 0; font-size: 1em; }

img, svg { max-width: 100%; height: auto; display: block; }

/* ─── Layout ─────────────────────────────────────────────────────────────── */
.page-wrapper {
  max-width: 860px;
  margin: 0 auto;
  padding: 0 1.5rem;
}

.wide-wrapper {
  max-width: 1040px;
  margin: 0 auto;
  padding: 0 1.5rem;
}

.split {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 2.5rem;
  align-items: start;
}

.split-3 {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 1rem;
}

@media (max-width: 680px) {
  .split   { grid-template-columns: 1fr; }
  .split-3 { grid-template-columns: 1fr; }
}

/* ─── Typography ─────────────────────────────────────────────────────────── */
h1 { font-size: 2rem;   font-weight: 700; line-height: 1.2; letter-spacing: -0.02em; }
h2 { font-size: 1.4rem; font-weight: 600; line-height: 1.3; margin-top: 2.5rem; margin-bottom: 0.6rem; }
h3 { font-size: 1.05rem; font-weight: 600; line-height: 1.4; margin-top: 1.75rem; margin-bottom: 0.4rem; }

.lesson-subtitle { font-size: 1.05rem; color: var(--text-secondary); margin-top: 0.5rem; }
.small           { font-size: 0.82rem; color: var(--text-secondary); }
.muted           { color: var(--text-tertiary); }

p { margin-bottom: 0.9rem; }
p:last-child { margin-bottom: 0; }

ul, ol { margin-bottom: 0.9rem; padding-left: 1.5rem; }
li { margin-bottom: 0.35rem; }

/* ─── Site Header ────────────────────────────────────────────────────────── */
.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(17, 17, 19, 0.92);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--stroke-primary);
  padding: 0.7rem 1.5rem;
  display: flex;
  align-items: center;
  gap: 1.5rem;
}

.site-logo {
  font-weight: 700;
  font-size: 0.9rem;
  color: var(--text-primary);
  letter-spacing: -0.01em;
  white-space: nowrap;
  text-decoration: none;
}
.site-logo:hover { text-decoration: none; }

.breadcrumb {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 0.82rem;
  color: var(--text-tertiary);
  overflow: hidden;
}

.breadcrumb a              { color: var(--text-secondary); }
.breadcrumb a:hover        { color: var(--text-primary); text-decoration: none; }
.breadcrumb .bc-sep        { color: var(--text-tertiary); }
.breadcrumb .bc-current    { color: var(--text-primary); white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }

/* ─── Lesson Header ──────────────────────────────────────────────────────── */
.lesson-header {
  padding: 2.5rem 0 1.5rem;
  border-bottom: 1px solid var(--stroke-secondary);
  margin-bottom: 2.5rem;
}

.path-label {
  font-size: 0.73rem;
  font-weight: 700;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 0.6rem;
}

.progress-bar {
  margin-top: 1rem;
  height: 3px;
  background: var(--stroke-primary);
  border-radius: var(--r-full);
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  background: var(--accent);
  border-radius: var(--r-full);
}

/* ─── Lesson Section ─────────────────────────────────────────────────────── */
.lesson-section {
  padding: 2rem 0;
  border-bottom: 1px solid var(--stroke-secondary);
}

.lesson-section:last-of-type { border-bottom: none; }

/* ─── Cards ──────────────────────────────────────────────────────────────── */
.card {
  background: var(--bg-card);
  border: 1px solid var(--stroke-primary);
  border-radius: var(--r-lg);
  overflow: hidden;
  margin: 1rem 0;
}

.card-header {
  padding: 0.5rem 1rem;
  font-size: 0.72rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.07em;
  color: var(--text-tertiary);
  border-bottom: 1px solid var(--stroke-secondary);
}

.card-body { padding: 1rem; }
.card-body > p:last-child { margin-bottom: 0; }

/* ─── Callouts ───────────────────────────────────────────────────────────── */
.callout {
  border-left: 3px solid var(--accent);
  border-radius: 0 var(--r-md) var(--r-md) 0;
  padding: 1rem 1.1rem;
  margin: 1.2rem 0;
  background: rgba(59, 130, 246, 0.05);
}

.callout--info    { border-left-color: var(--accent); background: rgba(59,130,246,0.05); }
.callout--success { border-left-color: var(--cert-lean); background: rgba(34,197,94,0.05); }
.callout--warning { border-left-color: var(--cert-b); background: rgba(245,158,11,0.05); }
.callout--neutral { border-left-color: var(--stroke-primary); background: var(--bg-elevated); }

.callout-title {
  font-weight: 700;
  font-size: 0.9rem;
  margin-bottom: 0.4rem;
}

.callout > p:last-child { margin-bottom: 0; }

/* ─── Certification Badges ───────────────────────────────────────────────── */
.cert {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.01em;
  padding: 2px 8px;
  border-radius: var(--r-full);
  vertical-align: middle;
  white-space: nowrap;
}

.cert--lean { background: rgba(34,197,94,0.12); color: var(--cert-lean); border: 1px solid rgba(34,197,94,0.25); }
.cert--ad   { background: rgba(59,130,246,0.12); color: var(--cert-ad);   border: 1px solid rgba(59,130,246,0.25); }
.cert--b    { background: rgba(245,158,11,0.12); color: var(--cert-b);    border: 1px solid rgba(245,158,11,0.25); }

.cert sub              { line-height: 0; vertical-align: sub; }
.cert-source           { font-size: 0.66rem; opacity: 0.85; color: inherit; text-decoration: none; }
.cert-source:hover     { opacity: 1; }

/* ─── Step-through ───────────────────────────────────────────────────────── */
.steps { margin: 1.5rem 0; }

.steps-nav {
  display: flex;
  gap: 6px;
  flex-wrap: wrap;
  margin-bottom: 1rem;
}

.step-pill {
  padding: 5px 13px;
  border-radius: var(--r-full);
  font-size: 0.78rem;
  font-weight: 500;
  border: 1px solid var(--stroke-primary);
  background: transparent;
  color: var(--text-secondary);
  cursor: pointer;
  transition: all 0.14s;
  font-family: inherit;
}

.step-pill:hover              { border-color: var(--accent); color: var(--text-primary); }
.step-pill.active             { background: var(--accent); border-color: var(--accent); color: #fff; font-weight: 600; }

.step-panel  { display: none; }
.step-panel.active { display: block; }

.steps-footer {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  margin-top: 1rem;
  padding-top: 1rem;
  border-top: 1px solid var(--stroke-secondary);
}

.steps-counter { margin-left: auto; font-size: 0.78rem; color: var(--text-tertiary); }

/* ─── Buttons ────────────────────────────────────────────────────────────── */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  padding: 7px 16px;
  border-radius: var(--r-md);
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  border: 1px solid transparent;
  font-family: inherit;
  transition: opacity 0.14s, border-color 0.14s;
}

.btn:disabled        { opacity: 0.3; cursor: not-allowed; }
.btn--primary        { background: var(--accent); color: #fff; }
.btn--primary:hover:not(:disabled) { opacity: 0.85; }
.btn--secondary      { background: var(--bg-elevated); color: var(--text-primary); border-color: var(--stroke-primary); }
.btn--secondary:hover:not(:disabled) { border-color: var(--accent); }

/* ─── Worked Example ─────────────────────────────────────────────────────── */
.example-box {
  border-left: 3px solid var(--accent);
  background: var(--bg-elevated);
  border-radius: 0 var(--r-md) var(--r-md) 0;
  padding: 1rem 1.1rem;
  margin: 1rem 0;
}

.example-header {
  font-size: 0.7rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--accent);
  margin-bottom: 0.75rem;
}

.calc-table {
  width: 100%;
  border-collapse: collapse;
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.82rem;
  overflow-x: auto;
  display: block;
}

.calc-table th {
  text-align: center;
  padding: 5px 10px;
  border-bottom: 1px solid var(--stroke-primary);
  font-size: 0.7rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  color: var(--text-tertiary);
  white-space: nowrap;
}

.calc-table td {
  text-align: center;
  padding: 5px 10px;
  border-bottom: 1px solid var(--stroke-secondary);
  color: var(--text-primary);
  white-space: nowrap;
}

.calc-table tr:last-child td { border-bottom: none; }
.calc-table td strong        { color: var(--accent); font-weight: 700; }

/* ─── Data Tables ────────────────────────────────────────────────────────── */
.data-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.9rem;
  margin: 1rem 0;
  border: 1px solid var(--stroke-primary);
  border-radius: var(--r-md);
  overflow: hidden;
}

.data-table th {
  text-align: left;
  padding: 8px 14px;
  background: var(--bg-elevated);
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-tertiary);
  border-bottom: 1px solid var(--stroke-primary);
}

.data-table td {
  padding: 9px 14px;
  border-bottom: 1px solid var(--stroke-secondary);
  vertical-align: top;
  font-size: 0.88rem;
}

.data-table tr:last-child td  { border-bottom: none; }
.data-table tr:hover td       { background: rgba(255,255,255,0.02); }
.data-table .col-key          { font-family: 'JetBrains Mono', monospace; font-size: 0.8rem; color: var(--accent); font-weight: 600; }

/* ─── SVG / Diagrams ─────────────────────────────────────────────────────── */
.diagram-wrap {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.75rem;
  margin: 1.5rem 0;
}

.diagram-caption {
  font-size: 0.78rem;
  color: var(--text-tertiary);
  text-align: center;
  max-width: 380px;
  line-height: 1.5;
}

.diagram-legend {
  display: flex;
  gap: 1.2rem;
  align-items: center;
  flex-wrap: wrap;
  justify-content: center;
}

.legend-item {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 0.78rem;
  color: var(--text-secondary);
}

.legend-dot {
  width: 12px;
  height: 12px;
  border-radius: 50%;
  flex-shrink: 0;
}

/* ─── Tab Switcher ───────────────────────────────────────────────────────── */
.tabs { margin: 1rem 0; }

.tab-bar {
  display: flex;
  gap: 2px;
  border-bottom: 1px solid var(--stroke-primary);
  margin-bottom: 1rem;
}

.tab-btn {
  padding: 7px 14px;
  font-size: 0.83rem;
  font-weight: 500;
  background: transparent;
  border: none;
  color: var(--text-secondary);
  cursor: pointer;
  border-bottom: 2px solid transparent;
  margin-bottom: -1px;
  transition: color 0.14s;
  font-family: inherit;
}

.tab-btn:hover       { color: var(--text-primary); }
.tab-btn.active      { color: var(--accent); border-bottom-color: var(--accent); font-weight: 600; }

.tab-panel           { display: none; }
.tab-panel.active    { display: block; }

/* ─── Collapsible ────────────────────────────────────────────────────────── */
.collapsible-trigger {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  cursor: pointer;
  font-size: 0.85rem;
  color: var(--accent);
  background: none;
  border: none;
  padding: 0.4rem 0;
  font-weight: 600;
  font-family: inherit;
}

.collapsible-chevron         { transition: transform 0.2s; font-style: normal; }
.collapsible-trigger.open .collapsible-chevron { transform: rotate(90deg); }

.collapsible-content         { display: none; padding-top: 0.5rem; }
.collapsible-content.open    { display: block; }

/* ─── Navigation Footer ──────────────────────────────────────────────────── */
.lesson-footer {
  display: flex;
  gap: 1rem;
  justify-content: space-between;
  align-items: center;
  padding: 2.5rem 0 3rem;
  margin-top: 2rem;
  border-top: 1px solid var(--stroke-secondary);
}

.nav-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  padding: 10px 20px;
  border-radius: var(--r-md);
  font-size: 0.88rem;
  font-weight: 600;
  border: 1px solid var(--stroke-primary);
  color: var(--text-primary);
  background: var(--bg-elevated);
  text-decoration: none;
  transition: border-color 0.14s;
}

.nav-btn:hover         { border-color: var(--accent); text-decoration: none; }
.nav-btn--next         { background: var(--accent); border-color: var(--accent); color: #fff; }
.nav-btn--next:hover   { opacity: 0.87; text-decoration: none; }

@media (max-width: 500px) {
  .lesson-footer        { flex-direction: column; align-items: stretch; }
  .nav-btn              { justify-content: center; }
}

/* ─── See Also ───────────────────────────────────────────────────────────── */
.see-also {
  background: var(--bg-elevated);
  border: 1px solid var(--stroke-primary);
  border-radius: var(--r-lg);
  padding: 1.25rem 1.5rem;
  margin: 2rem 0 1rem;
}

.see-also h2                { margin-top: 0; font-size: 0.9rem; color: var(--text-secondary); }
.see-also-group             { margin-top: 1rem; }
.see-also-label             { font-size: 0.68rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.08em; color: var(--text-tertiary); margin-bottom: 0.35rem; }

.source-list                { list-style: none; padding: 0; margin: 0; }
.source-list li             { margin-bottom: 0.3rem; font-size: 0.87rem; }

/* ─── Key Takeaways ──────────────────────────────────────────────────────── */
.takeaway-list              { list-style: none; padding: 0; margin: 0; }

.takeaway-list li {
  padding: 0.75rem 0 0.75rem 1.4rem;
  border-bottom: 1px solid var(--stroke-secondary);
  position: relative;
  font-size: 0.93rem;
  line-height: 1.6;
}

.takeaway-list li::before   { content: "→"; position: absolute; left: 0; color: var(--accent); font-weight: 700; }
.takeaway-list li:last-child { border-bottom: none; }

/* ─── Home Page ──────────────────────────────────────────────────────────── */
.hero {
  padding: 4rem 0 3rem;
  text-align: center;
}

.hero-eyebrow {
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 1rem;
}

.hero h1 {
  font-size: 2.8rem;
  letter-spacing: -0.03em;
  margin-bottom: 1rem;
}

@media (max-width: 560px) { .hero h1 { font-size: 2rem; } }

.hero-desc {
  font-size: 1.08rem;
  color: var(--text-secondary);
  max-width: 580px;
  margin: 0 auto 2rem;
  line-height: 1.7;
}

.hero-cta {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  background: var(--accent);
  color: #fff;
  padding: 11px 24px;
  border-radius: var(--r-md);
  font-weight: 700;
  font-size: 0.95rem;
  text-decoration: none;
  transition: opacity 0.14s;
}

.hero-cta:hover { opacity: 0.86; text-decoration: none; }

.home-section { padding: 3rem 0; border-top: 1px solid var(--stroke-secondary); }
.home-section h2 { margin-top: 0; margin-bottom: 1.5rem; }

.path-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 1rem;
}

@media (max-width: 600px) { .path-grid { grid-template-columns: 1fr; } }

.path-card {
  background: var(--bg-card);
  border: 1px solid var(--stroke-primary);
  border-left: 4px solid var(--card-color, var(--accent));
  border-radius: var(--r-lg);
  padding: 1.2rem 1.3rem;
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
  text-decoration: none;
  color: inherit;
  transition: background 0.14s, border-color 0.14s;
}

.path-card:hover { background: var(--bg-elevated); text-decoration: none; }

.path-card-label {
  font-size: 0.68rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.09em;
  color: var(--card-color, var(--accent));
}

.path-card h3 { margin: 0; font-size: 0.97rem; line-height: 1.3; }

.path-card-desc {
  font-size: 0.83rem;
  color: var(--text-secondary);
  line-height: 1.5;
  flex: 1;
}

.path-card-meta { font-size: 0.72rem; color: var(--text-tertiary); }

/* ─── Path Landing Page ──────────────────────────────────────────────────── */
.path-hero {
  padding: 2.5rem 0 1.5rem;
  border-bottom: 1px solid var(--stroke-secondary);
  margin-bottom: 2rem;
}

.lesson-list {
  display: flex;
  flex-direction: column;
  gap: 0.6rem;
  margin: 1rem 0 2rem;
}

.lesson-card {
  display: flex;
  align-items: flex-start;
  gap: 1rem;
  background: var(--bg-card);
  border: 1px solid var(--stroke-primary);
  border-radius: var(--r-lg);
  padding: 1rem 1.3rem;
  text-decoration: none;
  color: inherit;
  transition: background 0.14s, border-color 0.14s;
}

.lesson-card:hover        { background: var(--bg-elevated); border-color: var(--accent); text-decoration: none; }

.lesson-num {
  flex-shrink: 0;
  width: 30px;
  height: 30px;
  border-radius: 50%;
  background: rgba(255, 255, 255, 0.07);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 0.78rem;
  font-weight: 700;
  color: var(--text-secondary);
  margin-top: 2px;
}

.lesson-card-body         { flex: 1; }
.lesson-card-body h3      { margin: 0 0 3px; font-size: 0.93rem; }
.lesson-card-body p       { margin: 0; font-size: 0.8rem; color: var(--text-secondary); line-height: 1.5; }
.lesson-card-arrow        { color: var(--text-tertiary); align-self: center; font-size: 1rem; }

/* ─── Home Footer ────────────────────────────────────────────────────────── */
.site-footer {
  padding: 2rem 1.5rem;
  border-top: 1px solid var(--stroke-secondary);
  text-align: center;
  font-size: 0.8rem;
  color: var(--text-tertiary);
  margin-top: 3rem;
}

.site-footer a { color: var(--text-secondary); }

/* ─── Lesson Prev/Next Navigation ───────────────────────────────────────── */
.lesson-nav {
  display: flex;
  gap: 0.75rem;
  justify-content: space-between;
  margin: 3rem 0 1rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--stroke-secondary);
}

.lesson-nav-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  padding: 8px 18px;
  border-radius: var(--r-md);
  font-size: 0.85rem;
  font-weight: 600;
  background: var(--bg-elevated);
  border: 1.5px solid var(--stroke-primary);
  color: var(--text-secondary);
  text-decoration: none;
  transition: border-color 0.15s, color 0.15s;
}

.lesson-nav-btn:hover {
  border-color: var(--accent);
  color: var(--text-primary);
  text-decoration: none;
}

.lesson-nav-next {
  margin-left: auto;
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
}

.lesson-nav-next:hover { opacity: 0.85; color: #fff; }

/* ─── Lean / Certification Badge ─────────────────────────────────────────── */
.lean-badge {
  display: inline-flex;
  align-items: center;
  gap: 3px;
  font-size: 0.7rem;
  font-weight: 700;
  letter-spacing: 0.01em;
  padding: 1px 7px;
  border-radius: var(--r-full);
  vertical-align: middle;
  white-space: nowrap;
  background: rgba(34,197,94,0.12);
  color: var(--cert-lean);
  border: 1px solid rgba(34,197,94,0.25);
  margin-left: 4px;
}

/* ─── Cross-reference links ──────────────────────────────────────────────── */
.xref {
  font-weight: 600;
  color: var(--accent);
  border-bottom: 1px dashed currentColor;
  text-decoration: none;
}
.xref:hover { text-decoration: none; border-bottom-style: solid; }

/* ─── Step panel content wrapper ─────────────────────────────────────────── */
.step-panel-content {
  padding: 1rem 0;
}

.step-panel-content h4 {
  margin: 0 0 0.6rem;
  font-size: 1rem;
  color: var(--text-primary);
}

/* ─── Step navigation buttons ────────────────────────────────────────────── */
.step-nav-btn {
  display: inline-flex;
  align-items: center;
  padding: 6px 14px;
  border-radius: var(--r-md);
  font-size: 0.82rem;
  font-weight: 600;
  cursor: pointer;
  border: 1.5px solid var(--stroke-primary);
  background: var(--bg-elevated);
  color: var(--text-primary);
  font-family: inherit;
  transition: border-color 0.14s;
}
.step-nav-btn:hover:not(:disabled) { border-color: var(--accent); }
.step-nav-btn:disabled { opacity: 0.35; cursor: not-allowed; }