/* ============================================
   STUDIO PHYSICS — SHARED STYLES
   ============================================ */

:root {
  --bg: #14130f;
  --bg-elevated: #1b1a15;
  --bg-card: #201f1a;
  --border: #2e2c25;
  --text: #e8e4da;
  --text-dim: #a3a08f;
  --accent: #9caf88;       /* sage green */
  --accent-soft: #29331f;
  --accent-warn: #d8a978;  /* misconceptions */
  --accent-warn-soft: #332a1c;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  line-height: 1.6;
}

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

h1, h2, h3, h4 {
  font-family: var(--font-serif);
  font-weight: 600;
  line-height: 1.25;
  color: var(--text);
}

/* ---------- Header / Nav ---------- */

.site-header {
  position: sticky;
  top: 0;
  z-index: 50;
  background: rgba(11, 13, 18, 0.92);
  backdrop-filter: blur(6px);
  border-bottom: 1px solid var(--border);
}

.header-inner {
  max-width: 1100px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 24px;
}

.brand {
  font-family: var(--font-serif);
  font-size: 1.15rem;
  font-weight: 700;
  color: var(--text);
  letter-spacing: 0.01em;
}
.brand:hover { text-decoration: none; color: var(--accent); }

.main-nav a {
  color: var(--text-dim);
  margin-left: 22px;
  font-size: 0.95rem;
}
.main-nav a:hover { color: var(--accent); text-decoration: none; }

.nav-toggle {
  display: none;
  background: none;
  border: none;
  color: var(--text);
  font-size: 1.4rem;
  cursor: pointer;
}

@media (max-width: 640px) {
  .nav-toggle { display: block; }
  .main-nav {
    display: none;
    flex-direction: column;
    position: absolute;
    top: 56px;
    left: 0;
    right: 0;
    background: var(--bg-elevated);
    border-bottom: 1px solid var(--border);
    padding: 10px 24px 16px;
  }
  .main-nav.open { display: flex; }
  .main-nav a { margin: 8px 0; }
}

/* ---------- Hero ---------- */

.hero {
  max-width: var(--max-width);
  margin: 64px auto 40px;
  padding: 0 24px;
  text-align: center;
}

.hero h1 {
  font-size: 2.4rem;
  max-width: 640px;
  margin: 0 auto;
}

.hero-sub {
  color: var(--text-dim);
  font-size: 1.08rem;
  max-width: 560px;
  margin: 14px auto 0;
}

/* ---------- Unit grid (homepage) ---------- */

.unit-grid-section {
  max-width: 1100px;
  margin: 0 auto 60px;
  padding: 0 24px;
}

.section-label {
  font-size: 1rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--accent);
  font-family: var(--font-mono);
  margin: 40px 0 16px;
  border-bottom: 1px solid var(--border);
  padding-bottom: 10px;
}

.unit-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 16px;
}

.unit-card {
  position: relative;
  display: flex;
  flex-direction: column;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 20px 18px;
  color: var(--text);
  transition: border-color 0.15s ease, transform 0.15s ease;
}

.unit-card:hover {
  border-color: var(--accent);
  transform: translateY(-2px);
  text-decoration: none;
}

.unit-number {
  font-family: var(--font-mono);
  color: var(--text-dim);
  font-size: 0.85rem;
  margin-bottom: 6px;
}

.unit-title {
  font-family: var(--font-serif);
  font-size: 1.15rem;
  font-weight: 600;
  margin-bottom: 6px;
}

.unit-desc {
  color: var(--text-dim);
  font-size: 0.92rem;
  flex-grow: 1;
}

.unit-tag {
  align-self: flex-start;
  margin-top: 12px;
  font-family: var(--font-mono);
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  padding: 3px 8px;
  border-radius: 5px;
}

.sim-tag {
  background: var(--accent-soft);
  color: var(--accent);
}

/* ---------- Lesson page layout ---------- */

.lesson-main {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 40px 24px 100px;
}

.lesson-header {
  margin-bottom: 36px;
}

.lesson-eyebrow {
  font-family: var(--font-mono);
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--accent);
}

.lesson-header h1 {
  font-size: 2.1rem;
  margin: 8px 0 0;
}

.lesson-toc {
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 16px 20px;
  margin-bottom: 40px;
}

.lesson-toc p {
  margin: 0 0 8px;
  font-family: var(--font-mono);
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-dim);
}

.lesson-toc ol {
  margin: 0;
  padding-left: 20px;
  columns: 2;
  font-size: 0.94rem;
}

.lesson-toc a { color: var(--text); }
.lesson-toc a:hover { color: var(--accent); }

.concept-block {
  margin-bottom: 56px;
  padding-top: 8px;
  border-top: 1px solid var(--border);
}

.concept-block h2 {
  font-size: 1.55rem;
  margin-bottom: 18px;
}

.concept-block h3 {
  font-size: 1.1rem;
  color: var(--accent);
  margin: 28px 0 10px;
}

.concept-block p { margin: 0 0 14px; }

/* Definition / variable list */

.var-list {
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 14px 18px;
  margin: 16px 0;
  list-style: none;
}

.var-list li {
  margin: 6px 0;
  font-size: 0.95rem;
}

.var-list code {
  color: var(--accent);
  font-weight: 600;
  margin-right: 8px;
}

/* Equations */

.equation {
  background: var(--bg-elevated);
  border-left: 3px solid var(--accent);
  border-radius: 6px;
  padding: 14px 20px;
  margin: 18px 0;
  font-family: var(--font-mono);
  font-size: 1.05rem;
  overflow-x: auto;
}

/* Worked example */

.worked-example {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 20px 22px;
  margin: 18px 0;
}

.worked-example .label {
  font-family: var(--font-mono);
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--accent);
  display: block;
  margin-bottom: 10px;
}

.worked-example ol {
  padding-left: 20px;
}

.worked-example li { margin-bottom: 10px; }

/* Misconceptions */

.misconception {
  background: var(--accent-warn-soft);
  border: 1px solid #5a4a26;
  border-left: 3px solid var(--accent-warn);
  border-radius: 6px;
  padding: 14px 18px;
  margin: 14px 0;
}

.misconception .label {
  font-family: var(--font-mono);
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--accent-warn);
  display: block;
  margin-bottom: 6px;
}

/* Simulator embed placeholder */

.sim-embed {
  background: var(--bg-elevated);
  border: 1px dashed var(--border);
  border-radius: 10px;
  padding: 40px 20px;
  text-align: center;
  color: var(--text-dim);
  margin: 18px 0;
  font-family: var(--font-mono);
  font-size: 0.9rem;
}

/* Practice problems */

.practice-problem {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 16px 20px;
  margin: 14px 0;
  background: var(--bg-card);
}

.practice-problem p { margin: 0 0 10px; }

.reveal-btn {
  background: var(--accent-soft);
  color: var(--accent);
  border: none;
  border-radius: 6px;
  padding: 7px 14px;
  font-family: var(--font-mono);
  font-size: 0.82rem;
  cursor: pointer;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.reveal-btn:hover { background: var(--accent); color: #0b0d12; }

.answer {
  display: none;
  margin-top: 12px;
  padding-top: 12px;
  border-top: 1px solid var(--border);
  color: var(--text-dim);
}

.answer.shown { display: block; }

/* Lesson nav (prev/next) */

.lesson-pager {
  display: flex;
  justify-content: space-between;
  margin-top: 60px;
  padding-top: 20px;
  border-top: 1px solid var(--border);
  font-family: var(--font-mono);
  font-size: 0.9rem;
}

/* Footer */

.site-footer {
  text-align: center;
  color: var(--text-dim);
  font-size: 0.85rem;
  padding: 30px 24px 50px;
  font-family: var(--font-mono);
}

/* Stub notice for lighter-content pages */

.stub-notice {
  background: var(--bg-elevated);
  border: 1px dashed var(--border);
  border-radius: 8px;
  padding: 12px 16px;
  color: var(--text-dim);
  font-size: 0.88rem;
  margin-bottom: 30px;
}