/* ===== CSS VARIABLES ===== */
:root {
  --bg-primary: #0b0d17;
  --bg-secondary: #111327;
  --bg-card: rgba(17,19,39,0.85);
  --bg-card-hover: rgba(25,28,55,0.95);
  --text-primary: #e8eaed;
  --text-secondary: #9aa0b8;
  --text-muted: #6b7194;
  --accent-blue: #4f8cff;
  --accent-purple: #8b5cf6;
  --accent-pink: #ec4899;
  --accent-cyan: #22d3ee;
  --accent-orange: #f97316;
  --accent-green: #34d399;
  --accent-red: #f43f5e;
  --accent-yellow: #fbbf24;
  --gradient-hero: linear-gradient(135deg, #4f8cff 0%, #8b5cf6 50%, #ec4899 100%);
  --gradient-card: linear-gradient(135deg, rgba(79,140,255,0.08) 0%, rgba(139,92,246,0.08) 100%);
  --border-color: rgba(79,140,255,0.15);
  --border-glow: rgba(79,140,255,0.4);
  --shadow-card: 0 8px 32px rgba(0,0,0,0.4);
  --shadow-glow: 0 0 30px rgba(79,140,255,0.2);
  --radius: 16px;
  --radius-sm: 10px;
  --radius-xs: 6px;
  --nav-height: 64px;
  --font-main: 'Inter', -apple-system, system-ui, sans-serif;
  --font-display: 'Space Grotesk', 'Inter', sans-serif;
}

/* ===== RESET & BASE ===== */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; scroll-padding-top: calc(var(--nav-height) + 24px); }
body {
  font-family: var(--font-main);
  background: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.7;
  overflow-x: hidden;
  min-height: 100vh;
}

/* ===== STARFIELD CANVAS ===== */
#starfield {
  position: fixed; top: 0; left: 0; width: 100%; height: 100%;
  z-index: 0; pointer-events: none;
}

/* ===== PROGRESS BAR ===== */
#progress-bar {
  position: fixed; top: 0; left: 0; height: 3px; z-index: 1001;
  background: var(--gradient-hero);
  transition: width 0.15s linear;
  box-shadow: 0 0 12px rgba(79,140,255,0.6);
}

/* ===== NAVIGATION ===== */
nav {
  position: sticky; top: 64px; left: 0; right: 0; height: var(--nav-height);
  background: rgba(11,13,23,0.85); backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--border-color);
  z-index: 1000; display: flex; align-items: center; padding: 0 24px;
}
nav .brand {
  font-family: var(--font-display); font-weight: 700; font-size: 1.15rem;
  background: var(--gradient-hero); -webkit-background-clip: text; -webkit-text-fill-color: transparent;
  background-clip: text; margin-right: auto; white-space: nowrap;
}
nav .nav-links { display: flex; gap: 6px; align-items: center; flex-wrap: wrap; }
nav .nav-links a {
  color: var(--text-secondary); text-decoration: none; font-size: 0.82rem;
  font-weight: 500; padding: 6px 12px; border-radius: var(--radius-xs);
  transition: all 0.2s;
}
nav .nav-links a:hover, nav .nav-links a.active {
  color: var(--accent-blue); background: rgba(79,140,255,0.1);
}

/* ===== MAIN LAYOUT ===== */
main {
  position: relative; z-index: 1; max-width: 960px;
  margin: 0 auto; padding: calc(var(--nav-height) + 32px) 20px 60px;
}

/* ===== HERO ===== */
.hero {
  text-align: center; padding: 60px 20px 50px; position: relative;
}
.hero h1 {
  font-family: var(--font-display); font-size: clamp(2.2rem, 5vw, 3.2rem);
  font-weight: 800; background: var(--gradient-hero);
  -webkit-background-clip: text; -webkit-text-fill-color: transparent;
  background-clip: text; margin-bottom: 16px; letter-spacing: -0.5px;
}
.hero p {
  color: var(--text-secondary); font-size: 1.1rem; max-width: 600px; margin: 0 auto;
}

/* ===== SECTION ===== */
section { margin-bottom: 48px; }
section > .section-header {
  margin-bottom: 28px; padding-bottom: 16px;
  border-bottom: 1px solid var(--border-color);
}
section > .section-header h2 {
  font-family: var(--font-display); font-size: 1.65rem; font-weight: 700;
  background: var(--gradient-hero); -webkit-background-clip: text;
  -webkit-text-fill-color: transparent; background-clip: text;
  display: flex; align-items: center; gap: 12px;
}
section > .section-header h2 .icon {
  font-size: 1.4rem; -webkit-text-fill-color: initial;
}

/* ===== CARD ===== */
.card {
  background: var(--bg-card); border: 1px solid var(--border-color);
  border-radius: var(--radius); padding: 28px; margin-bottom: 24px;
  backdrop-filter: blur(12px); transition: all 0.3s;
  box-shadow: var(--shadow-card);
}
.card:hover {
  border-color: var(--border-glow);
  box-shadow: var(--shadow-card), var(--shadow-glow);
  transform: translateY(-2px);
}

/* ===== LEARNING OUTCOMES ===== */
.learning-outcomes {
  background: linear-gradient(135deg, rgba(34,211,238,0.08) 0%, rgba(79,140,255,0.08) 100%);
  border: 1px solid rgba(34,211,238,0.2); border-radius: var(--radius-sm);
  padding: 20px 24px; margin-bottom: 24px;
}
.learning-outcomes h4 {
  color: var(--accent-cyan); font-size: 0.9rem; font-weight: 600;
  text-transform: uppercase; letter-spacing: 0.5px; margin-bottom: 10px;
}
.learning-outcomes ul { padding-left: 20px; }
.learning-outcomes li {
  color: var(--text-secondary); font-size: 0.92rem; margin-bottom: 6px;
}
.learning-outcomes li::marker { color: var(--accent-cyan); }

/* ===== KEY CONCEPT / FORMULA ===== */
.key-concept {
  background: linear-gradient(135deg, rgba(139,92,246,0.1) 0%, rgba(79,140,255,0.1) 100%);
  border-left: 4px solid var(--accent-purple); border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  padding: 20px 24px; margin: 20px 0; position: relative;
}
.key-concept .label {
  color: var(--accent-purple); font-size: 0.78rem; font-weight: 700;
  text-transform: uppercase; letter-spacing: 1px; margin-bottom: 8px;
}
.key-concept ul { padding-left: 18px; margin-top: 8px; }
.key-concept li { color: var(--text-secondary); margin-bottom: 6px; font-size: 0.95rem; }
.key-concept li strong { color: var(--text-primary); }

/* ===== EXAMPLE BOX ===== */
.example-box {
  background: linear-gradient(135deg, rgba(249,115,22,0.08) 0%, rgba(251,191,36,0.08) 100%);
  border-left: 4px solid var(--accent-orange); border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  padding: 18px 24px; margin: 20px 0;
}
.example-box .label {
  color: var(--accent-orange); font-size: 0.78rem; font-weight: 700;
  text-transform: uppercase; letter-spacing: 1px; margin-bottom: 6px;
}

/* ===== EVOLUTION STAGE ===== */
.evo-stage {
  border-left: 3px solid var(--accent-purple);
  padding: 20px 24px; margin: 16px 0; margin-left: 12px;
  position: relative; transition: all 0.3s;
}
.evo-stage::before {
  content: ''; position: absolute; left: -9px; top: 24px;
  width: 14px; height: 14px; border-radius: 50%;
  background: var(--accent-purple); border: 3px solid var(--bg-primary);
  box-shadow: 0 0 10px rgba(139,92,246,0.5);
}
.evo-stage h4 {
  font-family: var(--font-display); font-size: 1.1rem; font-weight: 600;
  color: var(--text-primary); margin-bottom: 8px;
}
.evo-stage p { color: var(--text-secondary); font-size: 0.95rem; }

/* ===== SIMULATION CONTAINERS ===== */
.sim-container {
  background: rgba(0,0,0,0.3); border: 1px solid var(--border-color);
  border-radius: var(--radius-sm); padding: 24px; margin: 24px 0;
  text-align: center; position: relative; overflow: hidden;
}
.sim-container h3 {
  font-family: var(--font-display); font-size: 1.1rem; font-weight: 600;
  color: var(--accent-cyan); margin-bottom: 16px; text-align: left;
}
.sim-container canvas {
  border-radius: var(--radius-xs); display: block; margin: 0 auto;
  max-width: 100%;
}
.sim-controls {
  display: flex; flex-wrap: wrap; gap: 12px; align-items: center;
  justify-content: center; margin-top: 16px;
}
.sim-controls label {
  color: var(--text-secondary); font-size: 0.85rem; font-weight: 500;
}
.sim-controls input[type="range"] {
  -webkit-appearance: none; appearance: none; height: 6px;
  background: rgba(79,140,255,0.2); border-radius: 3px; outline: none;
  cursor: pointer; width: 200px;
}
.sim-controls input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none; width: 18px; height: 18px; border-radius: 50%;
  background: var(--accent-blue); cursor: pointer;
  box-shadow: 0 0 10px rgba(79,140,255,0.5);
}
.sim-controls input[type="range"]::-moz-range-thumb {
  width: 18px; height: 18px; border-radius: 50%; border: none;
  background: var(--accent-blue); cursor: pointer;
}
.sim-info {
  color: var(--text-secondary); font-size: 0.85rem; margin-top: 10px;
  font-style: italic;
}

/* ===== BUTTONS ===== */
.btn {
  display: inline-flex; align-items: center; gap: 8px;
  padding: 10px 22px; border-radius: var(--radius-xs); border: none;
  font-family: var(--font-main); font-size: 0.9rem; font-weight: 600;
  cursor: pointer; transition: all 0.25s; color: #fff;
}
.btn-primary {
  background: var(--gradient-hero);
  box-shadow: 0 4px 15px rgba(79,140,255,0.3);
}
.btn-primary:hover {
  transform: translateY(-2px);
  box-shadow: 0 6px 20px rgba(79,140,255,0.45);
}
.btn-secondary {
  background: rgba(79,140,255,0.15); border: 1px solid rgba(79,140,255,0.3);
  color: var(--accent-blue);
}
.btn-secondary:hover { background: rgba(79,140,255,0.25); }
.btn-secondary.active {
  background: var(--accent-blue); color: #fff;
}

/* ===== STAGE NAV BUTTONS ===== */
.stage-nav { display: flex; gap: 8px; flex-wrap: wrap; margin-bottom: 16px; }
.stage-btn {
  padding: 8px 16px; border-radius: 20px; border: 1px solid var(--border-color);
  background: transparent; color: var(--text-secondary); font-size: 0.82rem;
  font-weight: 500; cursor: pointer; transition: all 0.25s;
  font-family: var(--font-main);
}
.stage-btn:hover { border-color: var(--accent-purple); color: var(--accent-purple); }
.stage-btn.active {
  background: var(--accent-purple); border-color: var(--accent-purple);
  color: #fff; box-shadow: 0 0 15px rgba(139,92,246,0.3);
}

/* ===== QUIZ ===== */
.quiz-container {
  background: linear-gradient(135deg, rgba(139,92,246,0.06) 0%, rgba(236,72,153,0.06) 100%);
  border: 1px solid rgba(139,92,246,0.2); border-radius: var(--radius);
  padding: 28px;
}
.quiz-q {
  background: rgba(0,0,0,0.25); border: 1px solid var(--border-color);
  border-radius: var(--radius-sm); padding: 20px; margin-bottom: 16px;
  transition: all 0.3s;
}
.quiz-q.correct { border-color: var(--accent-green); background: rgba(52,211,153,0.08); }
.quiz-q.incorrect { border-color: var(--accent-red); background: rgba(244,63,94,0.08); }
.quiz-q p.q-text {
  font-weight: 600; color: var(--text-primary); margin-bottom: 12px; font-size: 0.95rem;
}
.quiz-q label {
  display: flex; align-items: center; gap: 10px; padding: 8px 12px;
  border-radius: var(--radius-xs); cursor: pointer; transition: all 0.2s;
  color: var(--text-secondary); font-size: 0.9rem;
}
.quiz-q label:hover { background: rgba(79,140,255,0.08); color: var(--text-primary); }
.quiz-q input[type="radio"] {
  appearance: none; -webkit-appearance: none; width: 18px; height: 18px;
  border: 2px solid var(--text-muted); border-radius: 50%; cursor: pointer;
  transition: all 0.2s; flex-shrink: 0;
}
.quiz-q input[type="radio"]:checked {
  border-color: var(--accent-blue); background: var(--accent-blue);
  box-shadow: inset 0 0 0 3px var(--bg-primary);
}
.quiz-feedback {
  margin-top: 8px; padding: 10px 14px; border-radius: var(--radius-xs);
  font-size: 0.85rem; font-weight: 500; display: none;
}
.quiz-feedback.show { display: block; }
.quiz-feedback.correct-fb { background: rgba(52,211,153,0.12); color: var(--accent-green); }
.quiz-feedback.incorrect-fb { background: rgba(244,63,94,0.12); color: var(--accent-red); }
.quiz-score {
  text-align: center; padding: 24px; margin-top: 16px;
  border-radius: var(--radius-sm); display: none;
}
.quiz-score.show { display: block; }
.quiz-score .score-num {
  font-family: var(--font-display); font-size: 2.5rem; font-weight: 800;
  background: var(--gradient-hero); -webkit-background-clip: text;
  -webkit-text-fill-color: transparent; background-clip: text;
}
.quiz-score .score-bar {
  width: 100%; height: 8px; background: rgba(255,255,255,0.1);
  border-radius: 4px; margin: 12px 0; overflow: hidden;
}
.quiz-score .score-bar .fill {
  height: 100%; border-radius: 4px; background: var(--gradient-hero);
  transition: width 1s ease-out;
}

/* ===== HR DIAGRAM ===== */
.hr-tooltip {
  position: absolute; background: rgba(11,13,23,0.95); border: 1px solid var(--accent-blue);
  border-radius: var(--radius-xs); padding: 10px 14px; pointer-events: none;
  font-size: 0.8rem; color: var(--text-primary); display: none; z-index: 10;
  backdrop-filter: blur(8px); white-space: nowrap;
}


/* ===== RESPONSIVE ===== */
@media(max-width:768px) {
  main { padding-left: 14px; padding-right: 14px; }
  .card { padding: 20px; }
  nav .nav-links a { font-size: 0.75rem; padding: 5px 8px; }
  .sim-controls input[type="range"] { width: 150px; }
  .stage-nav { gap: 6px; }
  .stage-btn { padding: 6px 12px; font-size: 0.78rem; }
}
@media(max-width:480px) {
  nav .nav-links { display: none; }
  .hero h1 { font-size: 1.8rem; }
}

/* ===== SCROLL REVEAL ===== */
.reveal-item {
  opacity: 0; transform: translateY(24px);
  transition: opacity 0.6s ease-out, transform 0.6s ease-out;
}
.reveal-item.visible { opacity: 1; transform: translateY(0); }

/* ===== MISC ===== */
p { color: var(--text-secondary); }
strong { color: var(--text-primary); font-weight: 600; }
a { color: var(--accent-blue); }
ul, ol { padding-left: 20px; }
li { margin-bottom: 4px; color: var(--text-secondary); }
li strong { color: var(--text-primary); }