:root {
  /* BRAND — keep variable names, unify to single EconTeen green */
  --primary-green: #37C260;
  --primary-green-dark: #37C260;
  --primary-green-light: #37C260;

  /* SURFACE & SHADOW */
  --bg: #F6F8FA;
  --card-bg: #FFFFFF;
  --shadow: 0 1px 3px rgba(16, 24, 40, 0.06);
  --shadow-lg: 0 6px 18px rgba(16, 24, 40, 0.08);
  --shadow-xl: 0 10px 28px rgba(16, 24, 40, 0.12);

  /* RADII */
  --radius: 12px;
  --radius-lg: 16px;

  /* GRAYS */
  --gray-50: #F9FAFB;
  --gray-100: #F1F5F9;
  --gray-200: #E2E8F0;
  --gray-300: #CBD5E1;
  --gray-600: #64748B;
  --gray-700: #334155;
  --gray-900: #0F172A;

  /* SEMANTIC (kept) */
  --success: #059669;
  --warning: #D97706;
  --danger: #DC2626;
}

/* RESET */
* { box-sizing: border-box; margin: 0; padding: 0; }

/* TYPOGRAPHY — EconTeen brand: Poppins throughout */
body {
  background: var(--bg);
  font-family: 'Poppins', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  color: var(--gray-900);
  line-height: 1.6;
}

/* Headline weight & tracking */
h1, h2, h3, h4, h5, h6,
.brand,
.lesson-hero h1,
.navbar,
.navbar .brand {
  font-family: 'Poppins', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  font-weight: 800 !important;
  letter-spacing: -0.3px;
}

/* Nav emphasis */
.nav-link,
.nav-btn {
  font-family: 'Poppins', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  font-weight: 600;
}

/* NAVBAR */
.navbar {
  background: var(--card-bg);
  border-bottom: 1px solid var(--gray-200);
  padding: 1rem 0;
  position: sticky;
  top: 0;
  z-index: 100;
  backdrop-filter: saturate(180%) blur(10px);
  box-shadow: var(--shadow);
}

.brand {
  font-weight: 800;
  font-size: 1.5rem;
  color: var(--primary-green);
  text-decoration: none;
}

.nav-links {
  display: flex;
  gap: 2rem;
  align-items: center;
}

.nav-link {
  color: var(--gray-600);
  text-decoration: none;
  padding: 0.5rem 0;
  border-bottom: 2px solid transparent;
  transition: color .18s ease, border-bottom-color .18s ease;
  position: relative;
}

.nav-link.active,
.nav-link:hover {
  color: var(--primary-green);
  border-bottom-color: var(--primary-green);
}

.nav-btn {
  background: var(--primary-green);
  color: #fff;
  padding: 0.58rem 1.1rem;
  border-radius: var(--radius);
  text-decoration: none;
  font-weight: 700;
  transition: transform .15s ease, box-shadow .15s ease, background .15s ease;
  box-shadow: var(--shadow);
}

.nav-btn:hover {
  background: var(--primary-green);
  color: #fff;
  transform: translateY(-1px);
  box-shadow: var(--shadow-lg);
}

/* LESSON HERO */
.lesson-hero {
  /* keep gradient syntax, but same color both ends (single-brand-green) */
  background: linear-gradient(135deg, var(--primary-green) 0%, var(--primary-green) 100%);
  color: #fff;
  padding: 4rem 0;
  margin-bottom: 3rem;
  position: relative;
  overflow: hidden;
}

.lesson-hero::before {
  content: '';
  position: absolute; inset: 0;
  background: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1000 100" fill="white" opacity="0.08"><polygon points="0,0 1000,0 1000,60 0,100"/></svg>');
  background-size: cover;
}

.lesson-hero .container { position: relative; z-index: 2; }

.lesson-hero h1 {
  font-size: 3rem;
  font-weight: 800;
  margin-bottom: 1rem;
  line-height: 1.1;
}

.lesson-meta {
  display: flex; gap: 1rem;
  margin-bottom: 1.5rem;
  opacity: 0.92;
}

.lesson-meta span {
  background: rgba(255,255,255,0.2);
  padding: 0.4rem 1rem;
  border-radius: 24px;
  font-size: 0.92rem;
  font-weight: 600;
  backdrop-filter: blur(8px);
}

.lesson-description {
  font-size: 1.1rem;
  opacity: 0.96;
  max-width: 700px;
  line-height: 1.7;
}

/* LAYOUT GRID */
.content-grid {
  display: grid;
  grid-template-columns: 1fr 320px;
  gap: 3rem;
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 1rem;
}

.main-content { display: flex; flex-direction: column; gap: 2rem; }

/* CARDS */
.content-card {
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
  overflow: hidden;
  transition: transform .18s ease, box-shadow .18s ease;
  border: 1px solid var(--gray-100);
}

.content-card:hover {
  box-shadow: var(--shadow-lg);
  transform: translateY(-2px);
}

.card-header {
  padding: 1.6rem 2rem 1rem 2rem;
  background: linear-gradient(135deg, var(--gray-50) 0%, var(--gray-100) 100%);
  border-bottom: 1px solid var(--gray-200);
}

.card-header h3 {
  margin: 0;
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--primary-green);
  display: flex; align-items: center; gap: 0.5rem;
}

.card-content { padding: 2rem; }

/* CALLOUTS */
.highlight-box {
  background: linear-gradient(135deg, #F8FAFC 0%, #EFF4F9 100%);
  border-left: 4px solid var(--primary-green);
  padding: 1.25rem 1.5rem;
  margin: 1.5rem 0;
  border-radius: 0 var(--radius) var(--radius) 0;
  box-shadow: var(--shadow);
}

.key-points {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 1.25rem;
  margin: 2rem 0;
}

.key-point {
  background: linear-gradient(135deg, var(--gray-50) 0%, #FFFFFF 100%);
  padding: 1.25rem 1.25rem 1.1rem;
  border-radius: var(--radius);
  border-left: 4px solid var(--primary-green);
  box-shadow: var(--shadow);
  transition: transform .15s ease, box-shadow .15s ease;
}

.key-point:hover { transform: translateY(-2px); box-shadow: var(--shadow-lg); }

.key-point h4 {
  margin: 0 0 0.6rem 0;
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--primary-green);
}

/* ACTIVITY */
.activity-box {
  background: linear-gradient(135deg, #FEFCE8 0%, #FEF3C7 100%);
  border: 2px solid #FDE047;
  border-radius: var(--radius-lg);
  padding: 1.6rem;
  margin: 1.6rem 0 2rem;
  box-shadow: var(--shadow);
}

.activity-box h4 {
  color: var(--warning);
  margin-bottom: 0.8rem;
  font-weight: 700;
  font-size: 1.1rem;
}

/* SIDEBAR */
.sidebar {
  display: flex; flex-direction: column; gap: 2rem;
  position: sticky; top: 120px; height: fit-content;
}

.progress-card {
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-lg);
  padding: 1.6rem;
  border: 1px solid var(--gray-100);
  transition: transform .18s ease, box-shadow .18s ease;
}

.progress-card:hover { transform: translateY(-2px); box-shadow: var(--shadow-xl); }

.progress-card h4 {
  margin-bottom: 0.9rem;
  color: var(--primary-green);
  font-weight: 700;
}

/* PROGRESS */
.progress-bar {
  width: 100%; height: 12px;
  background: var(--gray-200);
  border-radius: 6px; overflow: hidden;
  margin: 1rem 0;
  box-shadow: inset 0 1px 2px rgba(16,24,40,0.06);
}

.progress-fill {
  height: 100%;
  /* keep gradient syntax but collapse to same green both ends */
  background: linear-gradient(90deg, var(--primary-green) 0%, var(--primary-green) 100%);
  width: 25%;
  transition: width 0.5s ease;
  position: relative;
}

.progress-fill::after {
  content: '';
  position: absolute; inset: 0;
  background: linear-gradient(90deg, transparent 0%, rgba(255,255,255,0.25) 50%, transparent 100%);
  animation: shine 2.2s infinite;
}

@keyframes shine { 0% { transform: translateX(-100%); } 100% { transform: translateX(100%); } }

/* QUIZ */
.quiz-container {
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-xl);
  padding: 2.2rem;
  margin: 2.2rem 0;
  border: 1px solid var(--gray-100);
}

.quiz-question {
  font-size: 1.2rem;
  font-weight: 700;
  margin-bottom: 1.4rem;
  color: var(--gray-900);
  line-height: 1.45;
}

.quiz-options { display: flex; flex-direction: column; gap: 0.9rem; }

.quiz-option {
  background: var(--gray-50);
  border: 2px solid var(--gray-200);
  border-radius: var(--radius);
  padding: 1rem 1.1rem;
  cursor: pointer;
  transition: transform .12s ease, background .12s ease, border-color .12s ease, color .12s ease;
  font-weight: 500;
  position: relative;
}

.quiz-option:hover {
  background: var(--primary-green-light);
  border-color: var(--primary-green);
  transform: translateX(3px);
}

.quiz-option.selected {
  background: var(--primary-green-light);
  border-color: var(--primary-green);
  color: var(--primary-green);
  font-weight: 600;
}

.quiz-option.correct {
  background: #F0FDF4;
  border-color: var(--success);
  color: var(--success);
}

.quiz-option.incorrect {
  background: #FEF2F2;
  border-color: var(--danger);
  color: var(--danger);
}

/* BUTTONS */
.btn-primary {
  background: var(--primary-green);
  border: none;
  padding: 0.9rem 1.6rem;
  border-radius: var(--radius);
  font-weight: 700;
  font-size: 1rem;
  transition: transform .15s ease, box-shadow .15s ease, background .15s ease;
  box-shadow: var(--shadow);
  color: #fff;
}

.btn-primary:hover {
  background: var(--primary-green);
  transform: translateY(-2px);
  box-shadow: var(--shadow-lg);
}

/* LESSON NAV */
.lesson-nav {
  display: flex; justify-content: space-between; align-items: center;
  margin-top: 3rem; padding: 2rem 0;
  border-top: 2px solid var(--gray-200);
}

/* MODAL */
.calculator-modal {
  display: none;
  position: fixed; inset: 0;
  background: rgba(2, 6, 23, 0.42);
  z-index: 1000;
  backdrop-filter: blur(4px);
}

.modal-content {
  background: var(--card-bg);
  margin: 5% auto;
  padding: 1.8rem;
  border-radius: var(--radius-lg);
  max-width: 520px;
  box-shadow: var(--shadow-xl);
  border: 1px solid var(--gray-100);
}

.calculator-input {
  width: 100%;
  padding: 0.75rem;
  border: 2px solid var(--gray-200);
  border-radius: var(--radius);
  margin-bottom: 1rem;
  font-size: 1rem;
}

.calculator-input:focus { outline: none; border-color: var(--primary-green); }

.calculator-result {
  background: #F2FBF6; /* subtle tint without extra green var */
  padding: 1.2rem;
  border-radius: var(--radius);
  margin-top: 1rem;
  border-left: 4px solid var(--primary-green);
}

/* COMPOUND EXAMPLE */
.compound-example {
  background: #F8FAFB;
  border: 2px solid var(--gray-200);
  border-radius: var(--radius-lg);
  padding: 1.6rem;
  margin: 1.6rem 0;
  box-shadow: var(--shadow);
}

.compound-example h5 {
  color: var(--gray-700);
  margin-bottom: 0.8rem;
}

/* INLINE MATH EXAMPLE */
.math-example {
  background: var(--gray-50);
  padding: 0.9rem 1rem;
  border-radius: var(--radius);
  margin: 0.9rem 0;
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
  border-left: 3px solid var(--warning);
}

/* RESPONSIVE */
@media (max-width: 768px) {
  .content-grid { grid-template-columns: 1fr; gap: 2rem; }
  .sidebar { position: static; order: -1; }
  .lesson-hero h1 { font-size: 2.2rem; }
  .nav-links { display: none; }

  .key-points { grid-template-columns: 1fr; }

  .content-card,
  .progress-card,
  .quiz-container {
    margin-left: -1rem;
    margin-right: -1rem;
    border-radius: 0;
  }
}