/* CSS dùng chung (token, nav, footer, nút) — tách từ index.html cho Help Center. Đổi brand ở đây + index.html. */
  :root, :root[data-theme="dark"] {
    --c1: #EF5350;   /* red — the app accent */
    --c2: #FF8A65;   /* coral midpoint */
    --c3: #FFAB91;   /* peach warm */
    --grad: linear-gradient(115deg, #FF8A80 0%, #EF5350 48%, #D32F2F 100%);
    --grad-soft: linear-gradient(115deg, rgba(239,83,80,.18), rgba(255,138,101,.14), rgba(255,171,145,.12));

    --bg:   #0d0808;
    --bg2:  #140c0c;
    --bg3:  #1a1212;
    --card: rgba(255,255,255,.035);
    --card-solid: #1a1210;
    --border: rgba(255,255,255,.09);
    --border-strong: rgba(255,255,255,.16);
    --text: #faf0ea;
    --muted: #ad978a;
    --shadow: 0 24px 70px rgba(0,0,0,.5);
    --glow: 0 0 44px rgba(229,57,53,.32);

    /* category accents (match in-app Tools Hub) */
    --cat-organize: #EF5350;
    --cat-convert:  #FF7043;
    --cat-edit:     #FF8A65;
    --cat-sign:     #E57373;
    --cat-security: #D32F2F;
    --cat-optimize: #FF6E40;
  }
  :root[data-theme="light"] {
    --bg:   #fdf6f4;
    --bg2:  #ffffff;
    --bg3:  #fef0ec;
    --card: #ffffff;
    --card-solid: #ffffff;
    --border: rgba(42,12,12,.10);
    --border-strong: rgba(42,12,12,.18);
    --text: #24110b;
    --muted: #83655c;
    --shadow: 0 20px 50px rgba(80,20,20,.12);
    --glow: 0 0 40px rgba(229,57,53,.22);
  }

  * { margin: 0; padding: 0; box-sizing: border-box; }
  html { scroll-behavior: smooth; -webkit-text-size-adjust: 100%; }
  body {
    background: var(--bg); color: var(--text); overflow-x: hidden;
    font-family: -apple-system, BlinkMacSystemFont, 'SF Pro Display', 'Segoe UI', Roboto, sans-serif;
    line-height: 1.5; transition: background .4s ease, color .4s ease;
  }
  a { color: inherit; }
  ::selection { background: rgba(229,57,53,.35); color: #fff; }
  img { max-width: 100%; }

  .gradient-text {
    background: var(--grad); -webkit-background-clip: text; background-clip: text;
    -webkit-text-fill-color: transparent; color: transparent;
    background-size: 200% auto; animation: shimmer 6s linear infinite;
  }
  @keyframes shimmer { to { background-position: 200% center; } }

  /* ============ AMBIENT BACKGROUND ============ */
  .bg-fx { position: fixed; inset: 0; z-index: -2; overflow: hidden; pointer-events: none; }

  /* scroll progress */
  .progress { position: fixed; top: 0; left: 0; height: 3px; width: 0; z-index: 200; background: var(--grad); box-shadow: var(--glow); }

  /* ============ NAV ============ */
  nav {
    position: fixed; top: 0; left: 0; right: 0; z-index: 100;
    display: flex; align-items: center; justify-content: space-between;
    padding: 0 clamp(16px,5%,64px); height: 68px;
    background: color-mix(in srgb, var(--bg) 78%, transparent);
    backdrop-filter: blur(18px) saturate(140%); -webkit-backdrop-filter: blur(18px) saturate(140%);
    border-bottom: 1px solid var(--border);
  }
  .logo { display: flex; align-items: center; gap: 11px; text-decoration: none; font-weight: 800; }
  .logo-mark {
    width: 38px; height: 38px; border-radius: 11px;
    background: linear-gradient(135deg, #FF8A80, #EF5350 50%, #D32F2F);
    display: grid; place-items: center; box-shadow: 0 0 44px rgba(229,57,53,.32); position: relative; overflow: hidden;
    flex-shrink: 0;
  }
  .logo-mark svg { width: 30px; height: 30px; }
  .logo-text { font-size: 19px; letter-spacing: -.4px; }
  .logo-text b { font-weight: 800; }
  .logo-text .g { background: var(--grad); -webkit-background-clip: text; background-clip: text; -webkit-text-fill-color: transparent; }
  nav .links { display: flex; gap: 28px; list-style: none; }
  nav .links a { color: var(--muted); text-decoration: none; font-size: 14px; font-weight: 550; transition: color .2s; position: relative; }
  nav .links a:hover { color: var(--text); }
  nav .links a::after { content:''; position:absolute; left:0; bottom:-6px; height:2px; width:0; background: var(--grad); transition: width .25s; border-radius:2px; }
  nav .links a:hover::after { width:100%; }
  .nav-right { display: flex; align-items: center; gap: 12px; }
  .theme-btn {
    width: 38px; height: 38px; border-radius: 10px; border: 1px solid var(--border);
    background: var(--card); color: var(--text); cursor: pointer; font-size: 16px;
    display: grid; place-items: center; transition: border-color .2s, transform .2s;
  }
  .theme-btn:hover { border-color: var(--border-strong); transform: rotate(-12deg); }
  .btn {
    display: inline-flex; align-items: center; gap: 9px; text-decoration: none; cursor: pointer;
    font-weight: 650; border-radius: 12px; border: 1px solid transparent; transition: transform .2s, box-shadow .2s, background .2s;
    white-space: nowrap;
  }
  .btn-grad { background: var(--grad); color: #fff; box-shadow: var(--glow); }
  .btn-grad:hover { transform: translateY(-2px); box-shadow: 0 0 66px rgba(229,57,53,.5); }
  .btn-ghost { background: var(--card); color: var(--text); border-color: var(--border); }
  .btn-ghost:hover { border-color: var(--border-strong); background: var(--bg3); transform: translateY(-2px); }
  .btn.sm { padding: 9px 18px; font-size: 13.5px; }
  .btn.lg { padding: 16px 30px; font-size: 16px; border-radius: 14px; }
  .nav-burger { display: none; }

  /* ============ LAYOUT ============ */
  footer { background: var(--bg2); border-top: 1px solid var(--border); padding: 60px clamp(16px,5%,64px) 32px; }
  .foot-grid { max-width: 1180px; margin: 0 auto; display: grid; grid-template-columns: 2fr 1fr 1fr 1fr; gap: 44px; }
  .foot-brand p { font-size: 13.5px; color: var(--muted); line-height: 1.7; margin-top: 14px; max-width: 280px; }
  .foot-col h5 { font-size: 12px; font-weight: 750; letter-spacing: 1px; text-transform: uppercase; color: var(--muted); margin-bottom: 15px; }
  .foot-col ul { list-style: none; display: flex; flex-direction: column; gap: 11px; }
  .foot-col a { color: var(--text); text-decoration: none; font-size: 14px; opacity: .82; transition: opacity .2s, color .2s; }
  .foot-col a:hover { opacity: 1; color: var(--c1); }
  .foot-bottom { max-width: 1180px; margin: 44px auto 0; padding-top: 24px; border-top: 1px solid var(--border); display: flex; justify-content: space-between; flex-wrap: wrap; gap: 12px; font-size: 13px; color: var(--muted); }
  @media (max-width: 800px){ .foot-grid{ grid-template-columns: 1fr 1fr; } }
  @media (max-width: 820px){ nav .links { display: none; } }