/* ===========================
   CSS Custom Properties
   =========================== */
:root {
  /* Colors — contrast ratios meet WCAG 2.1 AA (≥4.5:1 for normal text, ≥3:1 for large text) */
  --color-bg: #ffffff;
  --color-surface: #f5f7fa;
  --color-text: #1a1a2e;
  /* on white: 15.4:1 */
  --color-text-muted: #4a4a68;
  /* on white: 7.2:1 */
  --color-primary: #2563eb;
  /* on white: 4.6:1 */
  --color-primary-hover: #1d4ed8;
  --color-border: #d1d5db;
  --color-focus: #2563eb;
  --color-nav-bg: #1a1a2e;
  --color-nav-text: #f5f7fa;
  /* on nav-bg: 14.2:1 */
  --color-hero-bg: #eef2ff;
  --color-footer-bg: #1a1a2e;
  --color-footer-text: #d1d5db;
  /* on footer-bg: 10.3:1 */

  /* Spacing */
  --space-xs: 0.25rem;
  --space-sm: 0.5rem;
  --space-md: 1rem;
  --space-lg: 1.5rem;
  --space-xl: 2rem;
  --space-2xl: 3rem;
  --space-3xl: 4rem;

  /* Typography */
  --font-family: system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
  --font-size-sm: 0.875rem;
  --font-size-base: 1rem;
  --font-size-lg: 1.25rem;
  --font-size-xl: 1.5rem;
  --font-size-2xl: 2rem;
  --font-size-3xl: 2.5rem;
  --line-height: 1.6;

  /* Layout */
  --nav-height: 3.5rem;
  --max-width: 1200px;
  --border-radius: 0.5rem;

  /* Breakpoints (for reference — used in @media queries) */
  /* --bp-tablet: 768px; */
  /* --bp-desktop: 1200px; */
}

/* ===========================
   CSS Reset
   =========================== */
*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
  scroll-padding-top: var(--nav-height);
}

body {
  font-family: var(--font-family);
  font-size: var(--font-size-base);
  line-height: var(--line-height);
  color: var(--color-text);
  background-color: var(--color-bg);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

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

ul {
  list-style: none;
}

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

a:hover {
  text-decoration: underline;
}

h1,
h2,
h3 {
  line-height: 1.2;
  color: var(--color-text);
}

h1 {
  font-size: var(--font-size-3xl);
}

h2 {
  font-size: var(--font-size-2xl);
  margin-bottom: var(--space-lg);
}

h3 {
  font-size: var(--font-size-lg);
}

/* ===========================
   Focus Styles (Req 8.3)
   =========================== */
:focus-visible {
  outline: 3px solid var(--color-focus);
  outline-offset: 2px;
  border-radius: 2px;
}

/* ===========================
   Navigation Bar (Req 2.1)
   =========================== */
.nav-bar {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: var(--nav-height);
  padding: 0 var(--space-lg);
  background-color: var(--color-nav-bg);
  color: var(--color-nav-text);
}

.nav-bar .site-name {
  font-size: var(--font-size-lg);
  font-weight: 700;
  color: var(--color-nav-text);
}

/* Logo text colors matching brand */
.logo-learn {
  color: #4A90D9;
}

.logo-system {
  color: #E8A020;
}

.logo-design {
  color: #4CAF50;
}

.logo-xyz {
  color: #1a1a2e;
}

/* In nav bar (dark bg), override .xyz to be light */
.nav-bar .logo-xyz {
  color: var(--color-nav-text);
}

/* Hamburger toggle button — visible only on mobile */
.menu-toggle {
  display: flex;
  flex-direction: column;
  justify-content: center;
  gap: 4px;
  background: none;
  border: none;
  cursor: pointer;
  padding: var(--space-sm);
  z-index: 110;
}

.hamburger-line {
  display: block;
  width: 22px;
  height: 2px;
  background-color: var(--color-nav-text);
  border-radius: 1px;
  transition: transform 0.3s ease, opacity 0.3s ease;
}

/* Animate to X when open */
.menu-toggle[aria-expanded="true"] .hamburger-line:nth-child(1) {
  transform: translateY(6px) rotate(45deg);
}

.menu-toggle[aria-expanded="true"] .hamburger-line:nth-child(2) {
  opacity: 0;
}

.menu-toggle[aria-expanded="true"] .hamburger-line:nth-child(3) {
  transform: translateY(-6px) rotate(-45deg);
}

/* Mobile nav: hidden off-screen by default */
#nav-links {
  display: flex;
  flex-direction: column;
  position: fixed;
  top: var(--nav-height);
  right: -100%;
  width: 260px;
  height: calc(100vh - var(--nav-height));
  background-color: var(--color-nav-bg);
  padding: var(--space-lg);
  gap: var(--space-xs);
  overflow-y: auto;
  transition: right 0.3s ease;
  z-index: 99;
}

#nav-links.open {
  right: 0;
}

#nav-links a {
  color: var(--color-nav-text);
  font-size: var(--font-size-sm);
  padding: var(--space-sm) var(--space-sm);
  border-radius: var(--border-radius);
  transition: background-color 0.2s ease;
  display: block;
}

#nav-links a:hover {
  background-color: rgba(255, 255, 255, 0.1);
  text-decoration: none;
}

#nav-links a:focus-visible {
  outline-color: #ffffff;
}

/* ===========================
   Hero Section
   =========================== */
#hero {
  margin-top: var(--nav-height);
  padding: var(--space-3xl) var(--space-lg);
  background-color: var(--color-hero-bg);
  text-align: center;
}

#hero h1 {
  margin-bottom: var(--space-md);
  font-size: clamp(1.5rem, 6vw, 2.5rem);
  word-break: break-word;
}

#hero .tagline {
  font-size: var(--font-size-lg);
  color: var(--color-text-muted);
  max-width: 600px;
  margin: 0 auto;
}

/* ===========================
   Filter Input
   =========================== */
#filter-input {
  display: block;
  width: 100%;
  max-width: 500px;
  margin: var(--space-xl) auto;
  padding: var(--space-sm) var(--space-md);
  font-family: var(--font-family);
  font-size: var(--font-size-base);
  color: var(--color-text);
  background-color: var(--color-bg);
  border: 2px solid var(--color-border);
  border-radius: var(--border-radius);
  transition: border-color 0.2s ease;
}

#filter-input:focus {
  border-color: var(--color-focus);
  outline: none;
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.2);
}

/* ===========================
   Main Content
   =========================== */
#content {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: var(--space-xl) var(--space-lg);
}

/* ===========================
   Footer
   =========================== */
footer {
  background-color: var(--color-footer-bg);
  color: var(--color-footer-text);
  text-align: center;
  padding: var(--space-xl) var(--space-lg);
  margin-top: var(--space-3xl);
}

footer a {
  color: var(--color-footer-text);
  font-weight: 600;
}

footer a:hover {
  color: #ffffff;
}

footer a:focus-visible {
  outline-color: #ffffff;
}

/* ===========================
   Topic Sections (Req 3.2)
   =========================== */
.topic-section {
  margin-bottom: var(--space-3xl);
}

.topic-section h2 {
  border-bottom: 2px solid var(--color-border);
  padding-bottom: var(--space-sm);
}

/* ===========================
   Resource Grid — Mobile-first (Req 5.1, 5.2)
   =========================== */
.resource-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: var(--space-lg);
}

/* ===========================
   Resource Card (Req 4.2)
   =========================== */
.resource-card {
  background-color: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--border-radius);
  padding: var(--space-lg);
  transition: box-shadow 0.2s ease, transform 0.2s ease;
}

.resource-card:hover {
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
  transform: translateY(-2px);
}

.resource-type-badge {
  display: inline-block;
  font-size: var(--font-size-sm);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--color-primary);
  background-color: rgba(37, 99, 235, 0.1);
  padding: var(--space-xs) var(--space-sm);
  border-radius: var(--border-radius);
  margin-bottom: var(--space-sm);
}

.resource-title {
  margin-bottom: var(--space-sm);
}

.resource-title a {
  color: var(--color-text);
  font-weight: 600;
}

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

.resource-description {
  font-size: var(--font-size-sm);
  color: var(--color-text-muted);
  line-height: var(--line-height);
}

/* ===========================
   No Results Message (Req 9.4)
   =========================== */
.no-results {
  text-align: center;
  padding: var(--space-3xl) var(--space-lg);
  color: var(--color-text-muted);
  font-size: var(--font-size-lg);
}

/* ===========================
   Responsive: Tablet — multi-column grid (Req 5.3)
   =========================== */
@media (min-width: 768px) {
  .resource-grid {
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  }
}

@media (min-width: 1024px) {

  /* Hide hamburger on desktop */
  .menu-toggle {
    display: none;
  }

  /* Show nav links inline */
  #nav-links {
    position: static;
    flex-direction: row;
    width: auto;
    height: auto;
    padding: 0;
    gap: var(--space-sm);
    background-color: transparent;
    overflow-y: visible;
  }

  #nav-links a {
    padding: var(--space-xs) var(--space-sm);
  }
}

/* ===========================
   Responsive: Desktop — ultra-wide container (Req 5.1)
   =========================== */
@media (min-width: 1200px) {
  #content {
    max-width: var(--max-width);
    margin-left: auto;
    margin-right: auto;
  }

  .resource-grid {
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  }
}