:root {
  --color-primary: #3B82F6;
  --color-primary-hover: #2563EB;
  --color-secondary: #10B981;
  --color-secondary-hover: #059669;
  --color-accent: #F59E0B;
  --color-accent-hover: #D97706;
  --color-bg: #0C0F1A;
  --color-bg-secondary: #111627;
  --color-card: #141825;
  --color-card-hover: #1A1F35;
  --color-border: #1E2433;
  --color-text: #F1F5F9;
  --color-text-secondary: #94A3B8;
  --color-text-muted: #8293A7;
  --color-danger: #EF4444;
  --color-success: #10B981;
  --color-warning: #F59E0B;
  --font-family: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 16px;
  --shadow-card: 0 4px 6px -1px rgba(0, 0, 0, 0.3), 0 2px 4px -2px rgba(0, 0, 0, 0.2);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.4), 0 4px 6px -4px rgba(0, 0, 0, 0.3);
  --transition: 200ms ease;
  --max-width: 1200px;
  --content-width: 720px;
  --header-height: 64px;
}

[data-theme="light"] {
  --color-bg: #F8FAFC;
  --color-bg-secondary: #F1F5F9;
  --color-card: #FFFFFF;
  --color-card-hover: #F8FAFC;
  --color-border: #E2E8F0;
  --color-text: #0F172A;
  --color-text-secondary: #475569;
  --color-text-muted: #94A3B8;
  --shadow-card: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -2px rgba(0, 0, 0, 0.06);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -4px rgba(0, 0, 0, 0.06);
}

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

html {
  scroll-behavior: smooth;
  -webkit-text-size-adjust: 100%;
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}

body {
  font-family: var(--font-family);
  font-size: 16px;
  line-height: 1.6;
  color: var(--color-text);
  background-color: var(--color-bg);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

main {
  flex: 1;
  padding-top: calc(var(--header-height) + 1.5rem);
  padding-bottom: 3rem;
}

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

a {
  color: var(--color-primary);
  text-decoration: none;
  transition: color var(--transition);
}

a:hover { color: var(--color-primary-hover); }

a:focus-visible, button:focus-visible, input:focus-visible, select:focus-visible, textarea:focus-visible {
  outline: 2px solid var(--color-primary);
  outline-offset: 2px;
}

h1, h2, h3, h4 {
  font-weight: 700;
  line-height: 1.25;
  color: var(--color-text);
}

h1 { font-size: 2rem; margin-bottom: 0.5rem; }
h2 { font-size: 1.5rem; margin-bottom: 0.5rem; }
h3 { font-size: 1.25rem; margin-bottom: 0.5rem; }
p { margin-bottom: 1rem; color: var(--color-text-secondary); }

ul, ol {
  list-style-position: inside;
  padding-left: 0;
}

.container {
  width: 100%;
  max-width: var(--max-width);
  margin: 0 auto;
  padding-left: 1rem;
  padding-right: 1rem;
}

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  border: 0;
}

/* Buttons */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 0.5rem;
  padding: 0.75rem 1.5rem;
  font-family: var(--font-family);
  font-size: 1rem;
  font-weight: 600;
  line-height: 1;
  border: none;
  border-radius: var(--radius-sm);
  cursor: pointer;
  transition: background-color var(--transition), transform var(--transition);
  white-space: nowrap;
}

.btn:active { transform: scale(0.98); }
.btn:disabled { opacity: 0.5; cursor: not-allowed; }

.btn-primary {
  background-color: var(--color-primary);
  color: #fff;
}
.btn-primary:hover { background-color: var(--color-primary-hover); color: #fff; }

.btn-secondary {
  background-color: var(--color-card);
  color: var(--color-text);
  border: 1px solid var(--color-border);
}
.btn-secondary:hover { background-color: var(--color-card-hover); }

.btn-sm { padding: 0.5rem 1rem; font-size: 0.875rem; }

.btn-icon {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 40px;
  height: 40px;
  padding: 0;
  background: none;
  border: none;
  border-radius: var(--radius-sm);
  color: var(--color-text-secondary);
  cursor: pointer;
  transition: color var(--transition), background-color var(--transition);
}
.btn-icon:hover {
  color: var(--color-text);
  background-color: var(--color-card);
}

/* Cards */
.card {
  background-color: var(--color-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 1.5rem;
  box-shadow: var(--shadow-card);
  transition: box-shadow var(--transition), border-color var(--transition);
}

.card:hover {
  box-shadow: var(--shadow-lg);
  border-color: var(--color-primary);
}

.card-link {
  display: block;
  text-decoration: none;
  color: inherit;
}

/* Form elements */
.form-group {
  margin-bottom: 1.25rem;
}

.form-label {
  display: block;
  font-size: 0.875rem;
  font-weight: 500;
  color: var(--color-text-secondary);
  margin-bottom: 0.375rem;
}

.form-input, .form-select {
  width: 100%;
  height: 48px;
  padding: 0 1rem;
  font-family: var(--font-family);
  font-size: 1rem;
  font-variant-numeric: tabular-nums;
  color: var(--color-text);
  background-color: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  transition: border-color var(--transition), box-shadow var(--transition);
}

.form-input:focus, .form-select:focus {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.15);
  outline: none;
}

.form-input::placeholder { color: var(--color-text-muted); }

/* Header */
.site-header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--header-height);
  background-color: var(--color-card);
  border-bottom: 1px solid var(--color-border);
  z-index: 100;
  backdrop-filter: blur(12px);
}

.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 100%;
}

.logo {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  text-decoration: none;
  color: var(--color-text);
  font-weight: 700;
  font-size: 1.25rem;
}

.logo-icon {
  font-size: 1.5rem;
  color: var(--color-primary);
}

.main-nav { display: none; }

.nav-list {
  display: flex;
  gap: 0.25rem;
  list-style: none;
}

.nav-link {
  display: block;
  padding: 0.5rem 1rem;
  font-size: 0.9375rem;
  font-weight: 500;
  color: var(--color-text-secondary);
  border-radius: var(--radius-sm);
  transition: color var(--transition), background-color var(--transition);
}
.nav-link:hover {
  color: var(--color-text);
  background-color: var(--color-bg-secondary);
}

.header-actions {
  display: flex;
  align-items: center;
  gap: 0.25rem;
}

.theme-toggle .icon-sun { display: none; }
.theme-toggle .icon-moon { display: block; }
[data-theme="light"] .theme-toggle .icon-sun { display: block; }
[data-theme="light"] .theme-toggle .icon-moon { display: none; }

@media (min-width: 768px) {
  .main-nav { display: block; }
  .menu-toggle { display: none; }
}

/* Mobile menu */
.main-nav.is-open {
  display: block;
  position: fixed;
  top: var(--header-height);
  left: 0;
  right: 0;
  background-color: var(--color-card);
  padding: 1rem;
  z-index: 200;
  overflow-y: auto;
  border-bottom: 1px solid var(--color-border);
  box-shadow: 0 4px 24px rgba(0,0,0,0.4);
}

.main-nav.is-open .nav-list {
  flex-direction: column;
  gap: 0;
}

.main-nav.is-open .nav-link {
  padding: 1rem 0.5rem;
  font-size: 1.125rem;
  border-bottom: 1px solid var(--color-border);
}

/* Breadcrumb */
.breadcrumb {
  font-size: 0.875rem;
  color: var(--color-text-muted);
  margin-bottom: 1.5rem;
}

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

.breadcrumb a:hover { color: var(--color-primary); }

.breadcrumb-sep {
  margin: 0 0.5rem;
  color: var(--color-text-muted);
}

/* Footer */
.site-footer {
  background-color: var(--color-card);
  border-top: 1px solid var(--color-border);
  padding: 3rem 0 1.5rem;
  margin-top: auto;
}

.footer-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 2rem;
  margin-bottom: 2rem;
}

@media (min-width: 768px) {
  .footer-grid { grid-template-columns: 2fr 1fr 1fr 1fr; }
}

.footer-heading {
  font-size: 1rem;
  font-weight: 600;
  margin-bottom: 1rem;
  color: var(--color-text);
}

.footer-desc {
  font-size: 0.875rem;
  color: var(--color-text-secondary);
  margin-bottom: 0;
}

.footer-links {
  list-style: none;
}

.footer-links li { margin-bottom: 0.5rem; }

.footer-links a {
  font-size: 0.875rem;
  color: var(--color-text-secondary);
}
.footer-links a:hover { color: var(--color-primary); }

.footer-bottom {
  border-top: 1px solid var(--color-border);
  padding-top: 1.5rem;
  text-align: center;
}

.footer-bottom p {
  font-size: 0.8125rem;
  color: var(--color-text-muted);
  margin-bottom: 0;
}

/* FAQ Accordion */
.faq-section {
  margin: 3rem 0;
}

.faq-section h2 {
  margin-bottom: 1.5rem;
}

.faq-item {
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  margin-bottom: 0.5rem;
  overflow: hidden;
}

.faq-item summary {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 1rem 1.25rem;
  font-weight: 500;
  color: var(--color-text);
  cursor: pointer;
  list-style: none;
  transition: background-color var(--transition);
}

.faq-item summary::-webkit-details-marker { display: none; }

.faq-item summary:hover { background-color: var(--color-bg-secondary); }

.faq-item summary::after {
  content: '+';
  font-size: 1.25rem;
  color: var(--color-text-muted);
  transition: transform var(--transition);
}

.faq-item[open] summary::after {
  content: '−';
}

.faq-answer {
  padding: 0.75rem 1.25rem 1rem;
  font-size: 0.9375rem;
  color: var(--color-text-secondary);
  line-height: 1.7;
}

/* Related tools */
.related-section {
  margin: 3rem 0;
}

.related-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 1rem;
}

.related-card {
  padding: 1.25rem;
}

.related-card h3 {
  font-size: 1rem;
  margin-bottom: 0.25rem;
}

.related-card p {
  font-size: 0.8125rem;
  color: var(--color-text-muted);
  margin-bottom: 0;
}

/* Ad slot */
.ad-slot {
  margin: 2rem 0;
  min-height: 100px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: var(--radius-sm);
}

/* Number display */
.number-display {
  font-variant-numeric: tabular-nums;
  font-weight: 700;
  font-size: 1.75rem;
  color: var(--color-text);
}

/* Formula display */
.formula {
  font-family: 'Courier New', Courier, monospace;
  background-color: var(--color-bg-secondary);
  padding: 1rem 1.5rem;
  border-radius: var(--radius-sm);
  border: 1px solid var(--color-border);
  overflow-x: auto;
  font-size: 0.9375rem;
  color: var(--color-text);
  margin: 1rem 0;
}

/* Print styles */
@media print {
  .site-header, .site-footer, .ad-slot, .theme-toggle, .menu-toggle,
  .calc-reset, .results-actions, .related-section { display: none !important; }

  body {
    background: #fff;
    color: #000;
  }

  main { padding-top: 0; }

  .card, .faq-item {
    border-color: #ccc;
    box-shadow: none;
  }

  .faq-item[open] .faq-answer { display: block; }
}