/* =========================================================
   STYLE HOME/BASE - TRỐNG ĐỒNG + NÂU ĐẤT (FULL COPY)
   - Nền trống đồng mờ
   - Header/hero tone nâu đất
   - Card đẹp + responsive
   - ✅ Khu "Các bài thi Kiến thức chung" nền nâu đất (exams-brown)
   ẢNH NỀN: assets/images/trongdongx.jpg
   (CSS nằm trong assets/css/ => url("../images/....") là đúng)
========================================================= */

:root {
  /* Primary - Xanh trí tuệ */
  --primary-50: #eff6ff;
  --primary-100: #dbeafe;
  --primary-200: #bfdbfe;
  --primary-300: #93c5fd;
  --primary-400: #60a5fa;
  --primary-500: #3b82f6;
  --primary-600: #2563eb;
  --primary-700: #1d4ed8;
  --primary-800: #1e40af;
  --primary-900: #1e3a8a;

  /* Bronze - Nâu đất */
  --bronze-50: #fffbeb;
  --bronze-100: #fef3c7;
  --bronze-200: #fde68a;
  --bronze-300: #fcd34d;
  --bronze-400: #fbbf24;
  --bronze-500: #f59e0b;
  --bronze-600: #d97706;
  --bronze-700: #b45309;
  --bronze-800: #92400e;
  --bronze-900: #78350f;

  /* Neutral */
  --gray-50: #f8fafc;
  --gray-100: #f1f5f9;
  --gray-200: #e2e8f0;
  --gray-300: #cbd5e1;
  --gray-400: #94a3b8;
  --gray-500: #64748b;
  --gray-600: #475569;
  --gray-700: #334155;
  --gray-800: #1e293b;
  --gray-900: #0f172a;

  /* Semantic */
  --bg: var(--gray-50);
  --text: var(--gray-900);
  --text-muted: var(--gray-600);
  --text-light: var(--gray-500);

  /* Surfaces */
  --surface-white: #ffffff;
  --surface-card: rgba(255, 255, 255, 0.94);
  --surface-header: rgba(253, 246, 227, 0.92);
  --surface-hero: rgba(253, 246, 227, 0.96);
  --surface-hover: rgba(248, 250, 252, 0.85);

  /* Borders */
  --border-light: rgba(226, 232, 240, 0.85);
  --border-medium: rgba(203, 213, 225, 0.85);
  --border-strong: rgba(148, 163, 184, 0.65);
  --border-bronze: rgba(180, 83, 9, 0.18);

  /* Shadows */
  --shadow-sm: 0 1px 2px rgba(0,0,0,0.05);
  --shadow-md: 0 6px 16px rgba(2,6,23,0.10);
  --shadow-lg: 0 12px 26px rgba(2,6,23,0.12);
  --shadow-xl: 0 20px 40px rgba(2,6,23,0.16);
  --shadow-bronze: 0 14px 30px rgba(180, 83, 9, 0.12);

  /* Radius */
  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 14px;
  --radius-xl: 18px;
  --radius-full: 9999px;

  /* Typography */
  --font-sans: system-ui, -apple-system, "Segoe UI", Roboto, "Inter", "Helvetica Neue", Arial, sans-serif;
  --font-display: var(--font-sans);

  /* Transitions */
  --transition-fast: 150ms cubic-bezier(0.4,0,0.2,1);
  --transition-base: 250ms cubic-bezier(0.4,0,0.2,1);

  /* Spacing */
  --space-xs: 4px;
  --space-sm: 8px;
  --space-md: 12px;
  --space-lg: 16px;
  --space-xl: 24px;
  --space-2xl: 32px;
  --space-3xl: 48px;

  /* Layout */
  --container: 1200px;
}

/* ================= RESET & BASE ================= */
*,
*::before,
*::after { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: var(--font-sans);
  color: var(--text);
  line-height: 1.6;
  min-height: 100vh;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;

  /* ✅ Nền trống đồng mờ (giảm opacity để nhìn thấy hình) */
  background:
    linear-gradient(rgba(248,250,252,0.72), rgba(248,250,252,0.72)),
    url("../images/trongdongx.jpg") center/cover no-repeat fixed;
}

img { max-width: 100%; height: auto; display:block; }
a { color: inherit; }
p { margin: 0 0 var(--space-md); }

.container {
  width: 100%;
  max-width: var(--container);
  margin: 0 auto;
  padding: 0 var(--space-lg);
}

/* ================= TYPOGRAPHY ================= */
h1,h2,h3,h4,h5,h6 {
  font-family: var(--font-display);
  margin: 0;
  line-height: 1.2;
  color: var(--gray-900);
}

h1 { font-size: 1.6rem; }
h2 { font-size: 2.4rem; letter-spacing: -0.02em; }
h3 { font-size: 1.6rem; letter-spacing: -0.01em; }

.small, small {
  font-size: 0.875rem;
  color: var(--text-muted);
}

/* ================= ACCESSIBILITY ================= */
:focus-visible {
  outline: 2px solid var(--primary-500);
  outline-offset: 2px;
  border-radius: var(--radius-sm);
}

/* ================= HEADER ================= */
.header {
  position: sticky;
  top: 0;
  z-index: 100;

  background: var(--surface-header);
  border-bottom: 1px solid var(--border-bronze);
  box-shadow: 0 6px 18px rgba(120,53,15,0.08);

  backdrop-filter: blur(10px) saturate(160%);
  -webkit-backdrop-filter: blur(10px) saturate(160%);

  transition: box-shadow var(--transition-base), background var(--transition-base);
}

.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-xl);
  padding: var(--space-md) 0;
}

/* Brand */
.brand {
  display: flex;
  align-items: center;
  gap: var(--space-md);
  text-decoration: none;
  transition: opacity var(--transition-fast);
}
.brand:hover { opacity: 0.95; }

.brand .logo {
  width: 48px;
  height: 48px;
  border-radius: var(--radius-lg);
  display: grid;
  place-items: center;
  background: linear-gradient(135deg, var(--bronze-200), var(--bronze-300));
  border: 1px solid rgba(180,83,9,0.20);
  box-shadow: var(--shadow-sm);
  transition: transform var(--transition-base);
}
.brand:hover .logo { transform: scale(1.05); }

.brand .logo i {
  color: var(--bronze-700);
  font-size: 1.4rem;
}

.brand-text { display: flex; flex-direction: column; }
.brand-text h1 {
  font-size: 1.35rem;
  background: linear-gradient(135deg, var(--bronze-800), var(--bronze-700));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}
.brand-text p {
  margin: 2px 0 0;
  font-size: 0.9rem;
  color: var(--gray-600);
}

/* ================= NAV (DESKTOP) ================= */
.nav {
  display: flex;
  align-items: center;
  gap: 10px;
  flex-wrap: wrap;
}

.nav-link {
  display: inline-flex;
  align-items: center;
  gap: 8px;

  padding: 10px 14px;
  border-radius: var(--radius-full);
  text-decoration: none;
  font-weight: 800;
  font-size: 0.95rem;

  color: var(--gray-700);
  background: rgba(255,255,255,0.70);
  border: 1px solid rgba(203,213,225,0.75);

  box-shadow: 0 1px 2px rgba(0,0,0,0.04);
  transition: transform var(--transition-fast),
              box-shadow var(--transition-fast),
              background var(--transition-fast),
              border-color var(--transition-fast),
              color var(--transition-fast);
}

.nav-link i { color: var(--bronze-700); font-size: 1rem; }

.nav-link:hover {
  background: rgba(255,251,235,0.95);
  border-color: rgba(180,83,9,0.30);
  color: var(--bronze-800);
  transform: translateY(-1px);
  box-shadow: 0 10px 22px rgba(120,53,15,0.10);
}

/* ================= USER / AUTH AREA ================= */
.userbox {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  padding: 10px 12px;
  border-radius: var(--radius-xl);
  background: rgba(255,255,255,0.78);
  border: 1px solid var(--border-light);
  box-shadow: var(--shadow-sm);
}

.avatar {
  width: 38px;
  height: 38px;
  border-radius: 50%;
  display: grid;
  place-items: center;
  background: linear-gradient(135deg, var(--primary-500), var(--primary-600));
  color: #fff;
  font-weight: 800;
  flex-shrink: 0;
}

/* ================= BUTTONS ================= */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 10px;

  padding: 12px 18px;
  border-radius: var(--radius-lg);
  text-decoration: none;
  font-weight: 800;
  font-size: 1rem;

  border: 2px solid transparent;
  cursor: pointer;
  user-select: none;

  transition: transform var(--transition-fast),
              box-shadow var(--transition-fast),
              background var(--transition-fast),
              border-color var(--transition-fast),
              color var(--transition-fast);
}
.btn:active { transform: translateY(1px) scale(0.99); }

.btn-primary {
  background: linear-gradient(135deg, var(--primary-500), var(--primary-600));
  color: #fff;
  box-shadow: 0 10px 22px rgba(59,130,246,0.25);
}
.btn-primary:hover {
  background: linear-gradient(135deg, var(--primary-600), var(--primary-700));
  box-shadow: 0 14px 28px rgba(59,130,246,0.32);
  transform: translateY(-1px);
}

.btn-outline {
  background: rgba(255,255,255,0.78);
  color: var(--gray-800);
  border-color: rgba(203,213,225,0.9);
}
.btn-outline:hover {
  background: rgba(255,251,235,0.95);
  border-color: rgba(180,83,9,0.30);
  color: var(--bronze-800);
  transform: translateY(-1px);
}

/* ================= HERO ================= */
.hero {
  padding: var(--space-3xl) 0;
  position: relative;
  overflow: hidden;
}

.hero::before{
  content:"";
  position:absolute;
  inset:0;
  background: linear-gradient(
    135deg,
    rgba(180, 83, 9, 0.10) 0%,
    rgba(120, 53, 15, 0.06) 45%,
    rgba(59, 130, 246, 0.04) 100%
  );
  z-index:-1;
}

.hero-card{
  border-radius: var(--radius-xl);
  padding: var(--space-2xl);
  background: var(--surface-hero);
  border: 1px solid var(--border-bronze);
  box-shadow: var(--shadow-bronze);
}

.hero-top{
  display:flex;
  justify-content: space-between;
  align-items:flex-start;
  gap: var(--space-xl);
  flex-wrap: wrap;
  margin-bottom: var(--space-xl);
}

.hero h2{
  font-size: 2.4rem;
  margin: 0 0 var(--space-md);
  background: linear-gradient(135deg, var(--bronze-800), var(--primary-700));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  display: inline-block;
}

.hero h2::after{
  content:"";
  display:block;
  width:100%;
  height:4px;
  margin-top: var(--space-sm);
  background: linear-gradient(90deg, var(--bronze-600), var(--primary-500));
  border-radius: var(--radius-full);
}

.hero p{
  max-width: 760px;
  font-size: 1.08rem;
  color: var(--gray-700);
  line-height: 1.75;
}

/* ================= SECTIONS ================= */
.section { padding: var(--space-2xl) 0; }

.section-title h3 { font-size: 1.6rem; margin: 0 0 var(--space-xs); }
.section-title p { margin: 0; color: var(--text-muted); font-size: 1rem; }

.grid{
  display:grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: var(--space-lg);
  margin-top: var(--space-lg);
}

/* Cards */
.card{
  background: var(--surface-card);
  border: 1px solid var(--border-light);
  border-radius: var(--radius-lg);
  padding: var(--space-lg);
  box-shadow: var(--shadow-sm);
  transition: transform var(--transition-base),
              box-shadow var(--transition-base),
              border-color var(--transition-base);
}
.card:hover{
  border-color: rgba(180,83,9,0.25);
  transform: translateY(-4px);
  box-shadow: var(--shadow-lg);
}

.topic{
  text-decoration:none;
  color: inherit;
  display:flex;
  flex-direction: column;
  height: 100%;
}

.topic-name{
  font-size: 1.1rem;
  font-weight: 900;
  color: var(--gray-800);
  margin-bottom: var(--space-sm);
  line-height: 1.35;
}

.topic-meta{
  font-size: 0.92rem;
  color: var(--gray-600);
  margin-bottom: var(--space-md);
  line-height: 1.5;
  flex-grow: 1;
}

.badge{
  display:inline-flex;
  align-items:center;
  gap: 8px;
  padding: 10px 12px;
  border-radius: var(--radius-full);
  font-size: 0.9rem;
  font-weight: 900;
  background: linear-gradient(135deg, var(--bronze-100), var(--bronze-200));
  color: var(--bronze-800);
  border: 1px solid rgba(180,83,9,0.22);
}

/* =========================
   ✅ NỀN NÂU ĐẤT RIÊNG CHO KHU "CÁC BÀI THI KTC"
   Dùng trong HTML: <section class="section exams-brown" id="exams">
========================= */
.exams-brown{
  background: linear-gradient(
    180deg,
    rgba(253,246,227,0.96),
    rgba(253,246,227,0.92)
  );
  border-top: 1px solid rgba(180,83,9,0.14);
  border-bottom: 1px solid rgba(180,83,9,0.14);
  padding-top: 28px;
  padding-bottom: 28px;
}

/* Nếu muốn nhìn như 1 khung lớn bo góc bao quanh */
.exams-brown .container{
  padding-top: 8px;
  padding-bottom: 8px;
}

.exams-brown .card.topic{
  background: rgba(255,255,255,0.92);
  border: 1px solid rgba(180,83,9,0.16);
  box-shadow: 0 10px 22px rgba(120,53,15,0.08);
}
.exams-brown .card.topic:hover{
  border-color: rgba(180,83,9,0.28);
  box-shadow: 0 16px 30px rgba(120,53,15,0.12);
}

/* ================= FOOTER ================= */
.footer{
  margin-top: var(--space-2xl);
  padding: var(--space-2xl) 0 var(--space-xl);
  background: linear-gradient(180deg, transparent, rgba(251,191,36,0.06));
  border-top: 1px solid var(--border-light);
}

/* ================= RESPONSIVE ================= */
@media (max-width: 768px){
  :root{
    --space-xl: 20px;
    --space-2xl: 28px;
    --space-3xl: 40px;
  }

  body{
    /* iOS đôi khi fixed + cover giật => bỏ fixed */
    background:
      linear-gradient(rgba(248,250,252,0.75), rgba(248,250,252,0.75)),
      url("../images/trongdongx.jpg") center/cover no-repeat;
  }

  .header-inner{ flex-direction: column; gap: var(--space-lg); }
  .nav{ display:none; }

  .userbox{ width:100%; justify-content:center; }

  .hero{ padding: var(--space-2xl) 0; }
  .hero-card{ padding: var(--space-xl); }
  .hero h2{ font-size: 2rem; }
  .hero p{ font-size: 1rem; }

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

@media (prefers-color-scheme: dark){
  :root{
    --bg: #0f172a;
    --text: #f1f5f9;
    --text-muted: #94a3b8;
    --surface-card: rgba(30,41,59,0.86);
    --surface-header: rgba(15,23,42,0.92);
    --surface-hero: rgba(30,41,59,0.90);
    --border-light: rgba(51,65,85,0.60);
    --border-medium: rgba(51,65,85,0.65);
    --border-bronze: rgba(148, 163, 184, 0.28);
  }

  body{
    background:
      linear-gradient(rgba(15,23,42,0.92), rgba(15,23,42,0.92)),
      url("../images/trongdongx.jpg") center/cover no-repeat fixed;
  }

  .nav-link{
    background: rgba(30,41,59,0.60);
    border-color: rgba(51,65,85,0.75);
    color: #e2e8f0;
  }
  .nav-link i{ color: var(--bronze-400); }

  .btn-outline{
    background: rgba(30,41,59,0.60);
    border-color: rgba(51,65,85,0.75);
    color: #e2e8f0;
  }

  .exams-brown{
    background: linear-gradient(180deg, rgba(30,41,59,0.86), rgba(30,41,59,0.82));
    border-color: rgba(51,65,85,0.60);
  }
}

@media (prefers-reduced-motion: reduce){
  * { transition: none !important; animation: none !important; }
}