:root {
  /* Apple Light Mode (Default) */
  --color-primary: #0066cc;
  --color-primary-focus: #0071e3;
  --color-primary-on-dark: #2997ff;
  
  --color-ink: #1d1d1f;
  --color-body: #1d1d1f;
  --color-body-muted: #86868b;
  
  --color-canvas: #ffffff;
  --color-canvas-parchment: #f5f5f7;
  --color-surface: #ffffff;
  --color-surface-pearl: #fafafc;
  
  --color-divider: #e0e0e0;
  --color-divider-soft: rgba(0, 0, 0, 0.04);
  
  --color-success: #34c759;
  --color-danger: #ff3b30;

  --shadow-product: rgba(0, 0, 0, 0.22) 3px 5px 30px 0;
  --shadow-soft: 0 4px 20px rgba(0, 0, 0, 0.05);

  /* Radii */
  --radius-xs: 5px;
  --radius-sm: 8px;
  --radius-md: 11px;
  --radius-lg: 18px;
  --radius-xl: 24px;
  --radius-pill: 9999px;
}

html.dark {
  /* Apple Dark Mode */
  --color-primary: #0066cc;
  --color-primary-on-dark: #2997ff;

  --color-ink: #ffffff;
  --color-body: #ffffff;
  --color-body-muted: #a1a1a6;
  
  --color-canvas: #000000;
  --color-canvas-parchment: #161618;
  --color-surface: #1c1c1e;
  --color-surface-pearl: #2c2c2e;

  --color-divider: #38383a;
  --color-divider-soft: rgba(255, 255, 255, 0.08);

  --shadow-product: rgba(0, 0, 0, 0.5) 3px 5px 30px 0;
  --shadow-soft: 0 4px 20px rgba(0, 0, 0, 0.3);
}

body {
  background-color: var(--color-canvas);
  color: var(--color-body);
  font-family: -apple-system, BlinkMacSystemFont, "SF Pro Text", "Helvetica Neue", sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  scroll-behavior: smooth;
  margin: 0;
  padding: 0;
  overflow-x: clip; 
  touch-action: pan-y;
  transition: background-color 0.3s ease, color 0.3s ease;
  font-size: 17px;
  line-height: 1.47;
  letter-spacing: -0.374px;
}

h1, h2, h3, h4, h5, h6, .display-text {
  font-family: -apple-system, BlinkMacSystemFont, "SF Pro Display", "Helvetica Neue", sans-serif;
  color: var(--color-ink);
  letter-spacing: -0.02em;
}

.apple-title-hero {
  font-size: 32px;
  font-weight: 600;
  line-height: 1.1;
  letter-spacing: -0.015em;
}

@media (min-width: 640px) {
  .apple-title-hero {
    font-size: 56px;
    line-height: 1.07;
  }
}

.apple-title-lg {
  font-size: 28px;
  font-weight: 600;
  line-height: 1.2;
}

@media (min-width: 640px) {
  .apple-title-lg {
    font-size: 40px;
    line-height: 1.1;
  }
}

.apple-title-md {
  font-size: 24px;
  font-weight: 600;
  line-height: 1.3;
}

@media (min-width: 640px) {
  .apple-title-md {
    font-size: 34px;
    line-height: 1.47;
  }
}

.apple-body-strong {
  font-weight: 600;
  font-size: 17px;
}

.apple-caption {
  font-size: 14px;
  line-height: 1.43;
  letter-spacing: -0.224px;
}

/* Glassmorphism utility */
.glass {
  background: rgba(255, 255, 255, 0.8);
  backdrop-filter: saturate(180%) blur(20px);
  -webkit-backdrop-filter: saturate(180%) blur(20px);
  border-bottom: 1px solid var(--color-divider-soft);
  transition: background-color 0.3s ease, border-color 0.3s ease;
}

html.dark .glass {
  background: rgba(22, 22, 24, 0.8);
}

@media (max-width: 768px) {
  .glass {
    backdrop-filter: none;
    -webkit-backdrop-filter: none;
    background: rgba(255, 255, 255, 0.98);
  }
  html.dark .glass {
    background: rgba(22, 22, 24, 0.98);
  }
}

/* Layout and components */
.apple-card {
  background-color: var(--color-surface);
  border: 1px solid var(--color-divider);
  border-radius: var(--radius-lg);
  box-shadow: none;
  transition: transform 0.2s ease, border-color 0.2s ease, background-color 0.3s ease;
}

.apple-card-elevated {
  background-color: var(--color-surface);
  border: 1px solid var(--color-divider-soft);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-soft);
  transition: transform 0.2s ease, border-color 0.2s ease, background-color 0.3s ease;
}

.apple-card-hover:hover {
  transform: translateY(-2px);
}

.apple-btn-primary {
  background-color: var(--color-primary);
  color: #ffffff;
  border-radius: var(--radius-pill);
  padding: 11px 22px;
  font-weight: 400;
  font-size: 17px;
  border: none;
  cursor: pointer;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  transition: transform 0.1s ease, background-color 0.2s ease;
}
.apple-btn-primary:active {
  transform: scale(0.95);
}

.apple-btn-utility {
  background-color: var(--color-surface-pearl);
  color: var(--color-ink);
  border: 1px solid var(--color-divider);
  border-radius: var(--radius-sm);
  padding: 8px 15px;
  font-size: 14px;
  font-weight: 400;
  cursor: pointer;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  transition: transform 0.1s ease, background-color 0.2s ease, border-color 0.2s ease;
}
.apple-btn-utility:active {
  transform: scale(0.95);
}

/* Navigation tabs */
.nav-tab {
  color: var(--color-body-muted);
  transition: color 0.2s ease, background-color 0.2s ease;
}
.nav-tab.active {
  color: var(--color-ink);
  background-color: var(--color-divider);
}
html.dark .nav-tab.active {
  color: var(--color-ink);
  background-color: var(--color-divider-soft);
}

/* Card 3D Flip Setup */
.perspective-1000 {
  perspective: 1000px;
}
.flip-card-inner {
  position: relative;
  width: 100%;
  height: 100%;
  transition: transform 0.4s cubic-bezier(0.4, 0, 0.2, 1);
  transform-style: preserve-3d;
  will-change: transform;
}
.flip-card:active .flip-card-inner {
  transform: scale(0.97);
  transition: transform 0.2s ease-out;
}
.flip-card.flipped .flip-card-inner {
  transform: rotateY(180deg);
}
.flip-card.flipped:active .flip-card-inner {
  transform: rotateY(180deg) scale(0.97);
}
.flip-card-front, .flip-card-back {
  position: absolute;
  width: 100%;
  height: 100%;
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  transform: translateZ(0);
  border-radius: var(--radius-xl);
  padding: 40px;
  display: flex;
  flex-direction: column;
  justify-content: flex-start;
  align-items: center;
  text-align: center;
  background-color: var(--color-canvas);
  border: 1px solid var(--color-divider);
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.05);
  touch-action: pan-y;
  overflow-y: auto;
  overscroll-behavior: contain;
  transition: border-color 0.3s ease, background-color 0.3s ease;
}
.flip-card-back {
  transform: rotateY(180deg);
  background-color: var(--color-surface);
  border: 1px solid var(--color-divider);
}

/* 3D Glare Effect */
.glare {
  position: absolute;
  inset: 0;
  border-radius: var(--radius-xl);
  background: radial-gradient(circle at 50% 50%, rgba(255,255,255,0.4) 0%, transparent 60%);
  opacity: 0;
  transition: opacity 0.3s ease;
  pointer-events: none;
  mix-blend-mode: overlay;
}
@media (max-width: 768px) {
  .glare { display: none !important; }
}
html.dark .glare {
  background: radial-gradient(circle at 50% 50%, rgba(255,255,255,0.15) 0%, transparent 60%);
}
.flip-card:hover .glare {
  opacity: 1;
}

/* Staggered Content Animations */
.flip-card-back .stagger-1,
.flip-card-back .stagger-2,
.flip-card-back .stagger-3 {
  opacity: 0;
  transform: translateY(10px);
  transition: all 0.4s ease;
}

.flip-card.flipped .flip-card-back .stagger-1 {
  opacity: 1;
  transform: translateY(0);
  transition-delay: 0.1s;
}
.flip-card.flipped .flip-card-back .stagger-2 {
  opacity: 1;
  transform: translateY(0);
  transition-delay: 0.15s;
}
.flip-card.flipped .flip-card-back .stagger-3 {
  opacity: 1;
  transform: translateY(0);
  transition-delay: 0.2s;
}

/* Scroll reveal */
.reveal-item {
  opacity: 0;
  transform: translateY(40px);
  transition: opacity 0.4s ease-out, transform 0.4s cubic-bezier(0.175, 0.885, 0.32, 1.275);
  will-change: transform, opacity;
}
.reveal-item.is-visible {
  opacity: 1;
  transform: translateY(0);
}

/* Scrollbar customization */
::-webkit-scrollbar {
  display: none;
}
::-webkit-scrollbar-thumb:hover {
  background: var(--color-body-muted);
}

/* Base button animations */
button {
  transition: transform 0.1s ease-in-out, background-color 0.2s ease, opacity 0.2s ease, border-color 0.2s ease, color 0.2s ease;
}
button:active {
  transform: scale(0.95);
}

/* Floating Top Anchor */
#top-anchor {
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.3s ease;
  will-change: opacity;
  background-color: var(--color-surface-pearl);
  color: var(--color-ink);
  border: 1px solid var(--color-divider);
}
#top-anchor.visible {
  opacity: 1;
  pointer-events: auto;
}

/* Theme Utilities */
.text-ink { color: var(--color-ink); }
.text-body { color: var(--color-body); }
.text-muted { color: var(--color-body-muted); }
.text-primary { color: var(--color-primary); }
html.dark .text-primary { color: var(--color-primary-on-dark); }
.bg-canvas { background-color: var(--color-canvas); }
.bg-parchment { background-color: var(--color-canvas-parchment); }
.bg-surface { background-color: var(--color-surface); }
.border-divider { border-color: var(--color-divider); }

/* specific for the topic grid cards */
.apple-topic-card {
  background-color: var(--color-surface);
  border: 1px solid var(--color-divider);
  border-radius: var(--radius-lg);
  padding: 24px;
  text-align: left;
  transition: transform 0.2s ease, border-color 0.2s ease, background-color 0.3s ease;
}
.apple-topic-card:hover {
  transform: translateY(-2px);
  border-color: var(--color-primary);
}

/* Mobile Safe Area for Top Navigation */
@supports(padding: max(0px)) {
  .nav-safe-padding {
    padding-top: max(1rem, env(safe-area-inset-top));
    padding-left: max(1.5rem, env(safe-area-inset-left));
    padding-right: max(1.5rem, env(safe-area-inset-right));
  }
}