:root {
  /* Color Palette - Athletic Theme */
  --bg-color: #F4F7F9; /* Light grey/blue background */
  --card-bg: #FFFFFF;
  --text-primary: #1A1C20;
  --text-secondary: #5F6B7A;
  --accent-primary: #007BFF; /* Electric Blue */
  --accent-hover: #0056b3;
  --accent-secondary: #00d2ff;
  --success: #10B981; /* Green */
  --warning: #F59E0B;
  --danger: #EF4444;
  --border-color: #E2E8F0;
  
  /* Zones Colors */
  --zone-1: #9CA3AF; /* Recovery - Grey */
  --zone-2: #3B82F6; /* Aerobic - Blue */
  --zone-3: #10B981; /* Tempo - Green */
  --zone-4: #F59E0B; /* Threshold - Orange */
  --zone-5: #EF4444; /* VO2 Max - Red */

  /* Typography & Spacing */
  --font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  --border-radius: 12px;
  --border-radius-sm: 8px;
  --spacing-xs: 0.5rem;
  --spacing-sm: 1rem;
  --spacing-md: 1.5rem;
  --spacing-lg: 2rem;
  --spacing-xl: 3rem;
  --transition: all 0.3s ease;
}

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

body {
  font-family: var(--font-family);
  background-color: var(--bg-color);
  color: var(--text-primary);
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
}

/* Header */
.app-header {
  text-align: center;
  padding: var(--spacing-xl) var(--spacing-sm) var(--spacing-lg);
  background-color: var(--card-bg);
  box-shadow: 0 2px 10px rgba(0,0,0,0.02);
  margin-bottom: var(--spacing-md);
}

.app-header h1 {
  font-size: 1.8rem;
  font-weight: 800;
  letter-spacing: -0.02em;
  color: var(--text-primary);
  margin-bottom: var(--spacing-xs);
}

.app-header .subtitle {
  color: var(--text-secondary);
  font-size: 1rem;
  font-weight: 400;
}

/* Main Container */
.app-container {
  max-width: 600px;
  margin: 0 auto;
  padding: 0 var(--spacing-sm);
}

/* Tabs */
.tabs {
  display: flex;
  background-color: var(--card-bg);
  border-radius: var(--border-radius);
  padding: 0.3rem;
  margin-bottom: var(--spacing-md);
  box-shadow: 0 4px 6px -1px rgba(0,0,0,0.05);
}

.tab-button {
  flex: 1;
  background: transparent;
  border: none;
  padding: 0.8rem 0.5rem;
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--text-secondary);
  border-radius: var(--border-radius-sm);
  cursor: pointer;
  transition: var(--transition);
  outline: none;
}

.tab-button:hover {
  color: var(--text-primary);
}

.tab-button.active {
  background-color: var(--bg-color);
  color: var(--text-primary);
  box-shadow: 0 1px 3px rgba(0,0,0,0.1);
}

/* Tab Content */
.tab-pane {
  display: none;
  animation: fadeIn 0.4s ease forwards;
}

.tab-pane.active {
  display: block;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(10px); }
  to { opacity: 1; transform: translateY(0); }
}

/* Cards */
.card {
  background-color: var(--card-bg);
  border-radius: var(--border-radius);
  padding: var(--spacing-lg);
  margin-bottom: var(--spacing-md);
  box-shadow: 0 4px 6px -1px rgba(0,0,0,0.05), 0 2px 4px -1px rgba(0,0,0,0.03);
}

.card h2 {
  font-size: 1.4rem;
  margin-bottom: var(--spacing-md);
  color: var(--text-primary);
}

.card h3 {
  font-size: 1.2rem;
  margin-bottom: var(--spacing-sm);
  text-align: center;
}

.highlight-card {
  text-align: center;
  background: linear-gradient(135deg, var(--card-bg), #f8fbff);
  border: 2px solid var(--accent-primary);
}

.highlight-card h3 {
  color: var(--text-secondary);
  text-transform: uppercase;
  font-size: 0.9rem;
  letter-spacing: 0.05em;
  margin-bottom: var(--spacing-xs);
}

/* Typography elements */
.big-metric {
  font-size: 3.5rem;
  font-weight: 800;
  color: var(--text-primary);
  line-height: 1;
  margin: var(--spacing-xs) 0;
  letter-spacing: -0.03em;
}

.metric-label {
  font-size: 1rem;
  color: var(--text-secondary);
  font-weight: 500;
}

.helper-text {
  font-size: 0.9rem;
  color: var(--text-secondary);
  margin-bottom: var(--spacing-md);
}

/* Forms */
.form-group {
  margin-bottom: var(--spacing-md);
}

.row-group {
  display: flex;
  gap: var(--spacing-sm);
}

.half-width {
  flex: 1;
}

label {
  display: block;
  font-size: 0.9rem;
  font-weight: 600;
  margin-bottom: 0.4rem;
  color: var(--text-primary);
}

.input-field {
  width: 100%;
  padding: 1rem;
  font-size: 1.1rem;
  border: 1px solid var(--border-color);
  border-radius: var(--border-radius-sm);
  background-color: var(--bg-color);
  color: var(--text-primary);
  font-family: inherit;
  transition: var(--transition);
  outline: none;
}

.input-field:focus {
  border-color: var(--accent-primary);
  box-shadow: 0 0 0 3px rgba(0, 123, 255, 0.1);
  background-color: var(--card-bg);
}

.input-with-hint {
  position: relative;
}

.small-hint {
  position: absolute;
  right: 1rem;
  top: 50%;
  transform: translateY(-50%);
  font-size: 0.8rem;
}

.hidden {
  display: none !important;
}

/* Buttons */
.btn-primary {
  width: 100%;
  background-color: var(--accent-primary);
  color: white;
  border: none;
  padding: 1rem;
  font-size: 1.1rem;
  font-weight: 600;
  border-radius: var(--border-radius-sm);
  cursor: pointer;
  transition: var(--transition);
  box-shadow: 0 4px 6px rgba(0, 123, 255, 0.2);
}

.btn-primary:hover {
  background-color: var(--accent-hover);
  transform: translateY(-1px);
}

.btn-primary:active {
  transform: translateY(1px);
}

.btn-text {
  background: none;
  border: none;
  color: var(--accent-primary);
  font-weight: 600;
  cursor: pointer;
}

.btn-text:hover {
  text-decoration: underline;
}

/* Tables */
.table-container {
  overflow-x: auto;
}

table {
  width: 100%;
  border-collapse: collapse;
  margin-top: var(--spacing-sm);
}

th, td {
  padding: 0.8rem;
  text-align: left;
  border-bottom: 1px solid var(--border-color);
}

th {
  font-weight: 600;
  font-size: 0.85rem;
  text-transform: uppercase;
  color: var(--text-secondary);
  letter-spacing: 0.05em;
}

td {
  font-size: 1rem;
  font-weight: 500;
}

.zone-indicator {
  display: inline-block;
  width: 12px;
  height: 12px;
  border-radius: 50%;
  margin-right: 8px;
}

/* Affiliate Boxes */
.affiliate-box {
  background-color: #f8fafc;
  border: 1px solid #e2e8f0;
  border-radius: var(--border-radius);
  padding: var(--spacing-md);
  margin-bottom: var(--spacing-md);
  display: flex;
  flex-direction: column;
  gap: var(--spacing-sm);
}

.affiliate-content h4 {
  font-size: 1rem;
  color: var(--accent-primary);
  margin-bottom: 0.3rem;
  display: flex;
  align-items: center;
}

.affiliate-content h4::before {
  content: '★';
  margin-right: 6px;
  font-size: 1.2rem;
}

.affiliate-content p {
  font-size: 0.95rem;
  color: var(--text-secondary);
}

.affiliate-action {
  display: flex;
  flex-direction: column;
  gap: var(--spacing-xs);
}

.btn-affiliate {
  display: inline-block;
  text-align: center;
  background-color: #ff9900; /* Amazon Orange */
  color: #111;
  text-decoration: none;
  font-weight: 600;
  padding: 0.8rem 1rem;
  border-radius: var(--border-radius-sm);
  transition: var(--transition);
  font-size: 0.95rem;
}

.btn-affiliate:hover {
  background-color: #e68a00;
}

.btn-affiliate.alt {
  background-color: var(--text-primary);
  color: white;
}
.btn-affiliate.alt:hover {
  background-color: #333;
}

/* Ad Placement Placeholders */
.ad-container {
  width: 100%;
  background-color: #e2e8f0;
  border: 1px dashed #cbd5e1;
  display: flex;
  justify-content: center;
  align-items: center;
  margin: var(--spacing-md) 0;
}

#ad-banner-top {
  height: 90px;
  max-width: 728px;
  margin: 0 auto;
  margin-bottom: var(--spacing-md);
}

#ad-inline-results {
  height: 250px;
}

#ad-footer {
  height: 90px;
  max-width: 728px;
  margin: 0 auto var(--spacing-md);
}

.ad-placeholder-text {
  color: #94a3b8;
  font-size: 0.9rem;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

/* Footer */
.app-footer {
  text-align: center;
  padding: var(--spacing-lg) var(--spacing-sm);
  margin-top: var(--spacing-xl);
  border-top: 1px solid var(--border-color);
}

.disclaimer {
  color: var(--text-secondary);
  font-size: 0.8rem;
}

.disclaimer p {
  margin-bottom: 0.3rem;
}

/* Mobile Adjustments */
@media (min-width: 768px) {
  .app-header h1 {
    font-size: 2.2rem;
  }
  .big-metric {
    font-size: 4rem;
  }
  .affiliate-box {
    flex-direction: row;
    align-items: center;
    justify-content: space-between;
  }
  .affiliate-content {
    flex: 1;
    padding-right: var(--spacing-md);
  }
  .affiliate-action {
    flex: 0 0 auto;
    min-width: 220px;
  }
}