:root {
    --green: #04aa6d;
    --green-dark: #037d52;
    --green-light: #e7f7f0;
    --green-mid: #b3e8d4;
    --sidebar-width: 270px;
    --ad-width: 200px;
    --topnav-h: 52px;
    --subnav-h: 42px;
    --header-h: 94px;
    --gray-50: #f9fafb;
    --gray-100: #f3f4f6;
    --gray-200: #e5e7eb;
    --gray-400: #9ca3af;
    --gray-600: #4b5563;
    --gray-800: #1f2937;
    --gray-900: #111827;
    --code-bg: #282c34;
    --code-text: #abb2bf;
    --shadow-sm: 0 1px 3px rgba(0,0,0,0.08);
    --shadow-md: 0 4px 12px rgba(0,0,0,0.10);
    --radius: 6px;
    --font-body: 'DM Sans', sans-serif;
    --font-heading: 'Lora', serif;
    --font-code: 'Source Code Pro', monospace;
  }

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

  body {
    font-family: var(--font-body);
    background: var(--gray-50);
    color: var(--gray-800);
    font-size: 15px;
    line-height: 1.7;
  }

  a { color: var(--green-dark); text-decoration: none; }
  a:hover { text-decoration: underline; }

  /* ─── NAVBAR WRAPPER ─── */
  .navbar-wrapper {
    position: fixed; top: 0; left: 0; right: 0; z-index: 1000;
    box-shadow: 0 2px 10px rgba(0,0,0,0.12);
  }

  /* ─── TOP NAV (Brand + Auth) ─── */
  .topnav {
    height: var(--topnav-h);
    background: #ffffff;
    display: flex; align-items: center;
    justify-content: space-between;
    padding: 0 28px;
    border-bottom: 1px solid var(--gray-200);
  }
  .topnav-logo {
    display: flex; align-items: center; gap: 12px;
    text-decoration: none;
  }
  .topnav-logo .logo-icon {
    width: 38px; height: 38px;
    background: linear-gradient(135deg, var(--green) 0%, var(--green-dark) 100%);
    border-radius: 10px;
    display: flex; align-items: center; justify-content: center;
    font-size: 1.15rem; flex-shrink: 0;
    box-shadow: 0 2px 8px rgba(4,170,109,0.3);
  }
  .topnav-logo .logo-text { display: flex; flex-direction: column; line-height: 1.15; }
  .topnav-logo .brand-name {
    font-family: var(--font-heading);
    font-size: 1.15rem; font-weight: 700;
    color: var(--gray-900); letter-spacing: -0.3px;
    white-space: nowrap;
  }
  .topnav-logo .brand-name span { color: var(--green); }
  .topnav-logo .brand-tagline {
    font-size: 0.68rem; color: var(--gray-400);
    font-weight: 500; letter-spacing: 0.3px;
  }
  .topnav-right {
    display: flex; align-items: center; gap: 10px;
  }
  .btn-outline {
    border: 1.5px solid var(--green); color: var(--green-dark);
    padding: 7px 20px; border-radius: 5px; font-size: 0.83rem;
    font-weight: 600; cursor: pointer; background: transparent;
    transition: all 0.15s; font-family: var(--font-body);
  }
  .btn-outline:hover { background: var(--green-light); }
  .btn-solid {
    background: var(--green); color: #fff;
    padding: 7px 20px; border-radius: 5px; font-size: 0.83rem;
    font-weight: 700; cursor: pointer; border: none;
    transition: background 0.15s; font-family: var(--font-body);
    box-shadow: 0 2px 6px rgba(4,170,109,0.3);
  }
  .btn-solid:hover { background: var(--green-dark); }
  .hamburger {
    display: none; background: none; border: none; cursor: pointer;
    padding: 4px 8px; color: var(--gray-800); font-size: 1.3rem;
  }
  .search-bar { display: none; }

  /* ─── SUBJECT NAV (Bottom bar) ─── */
  .subnav-wrapper {
    background: #037d52;
    height: var(--subnav-h);
    display: flex; align-items: center;
    position: relative;
    border-top: 1px solid rgba(255,255,255,0.12);
  }
  .subnav-scroll-btn {
    flex-shrink: 0;
    width: 32px; height: 100%;
    background: linear-gradient(to right, #037d52 60%, transparent);
    border: none; cursor: pointer;
    color: rgba(255,255,255,0.9);
    font-size: 1rem;
    display: flex; align-items: center; justify-content: center;
    z-index: 2;
    transition: background 0.15s;
    user-select: none;
  }
  .subnav-scroll-btn.right {
    background: linear-gradient(to left, #037d52 60%, transparent);
  }
  .subnav-scroll-btn:hover { color: #fff; }
  .subnav-scroll-btn.hidden { opacity: 0; pointer-events: none; }
  .subnav {
    flex: 1;
    height: 100%;
    display: flex; align-items: center;
    padding: 0 4px;
    overflow-x: auto;
    scrollbar-width: none;
    gap: 2px;
    scroll-behavior: smooth;
    -webkit-overflow-scrolling: touch;
  }
  .subnav::-webkit-scrollbar { display: none; }
  .subnav a {
    color: rgba(255,255,255,0.88);
    font-size: 0.82rem; font-weight: 500;
    padding: 5px 13px; border-radius: 4px;
    white-space: nowrap; transition: background 0.15s;
    text-decoration: none; flex-shrink: 0;
  }
  .subnav a:hover { background: rgba(255,255,255,0.15); color: #fff; text-decoration: none; }
  .subnav a.active {
    background: rgba(255,255,255,0.22); color: #fff; font-weight: 700;
    border-bottom: 2px solid #b3fde0;
  }
  .subnav-divider {
    width: 1px; height: 18px; background: rgba(255,255,255,0.2);
    flex-shrink: 0; margin: 0 4px;
  }

  /* ─── LAYOUT ─── */
  .layout-wrapper {
    display: flex;
    margin-top: var(--header-h);
    min-height: calc(100vh - var(--header-h));
  }

  /* ─── SIDEBAR ─── */
  .sidebar {
    width: var(--sidebar-width);
    min-width: var(--sidebar-width);
    background: #fff;
    border-right: 1px solid var(--gray-200);
    position: sticky; top: var(--header-h);
    height: calc(100vh - var(--header-h));
    overflow-y: auto;
    scrollbar-width: thin;
    scrollbar-color: var(--gray-200) transparent;
    flex-shrink: 0;
  }
  .sidebar::-webkit-scrollbar { width: 5px; }
  .sidebar::-webkit-scrollbar-thumb { background: var(--gray-200); border-radius: 3px; }

  .sidebar-section { padding: 0; }
  .sidebar-heading {
    font-size: 0.7rem; font-weight: 700; letter-spacing: 1.2px;
    text-transform: uppercase; color: var(--gray-400);
    padding: 18px 18px 6px;
  }
  .sidebar-link {
    display: flex; align-items: center; gap: 10px;
    padding: 8px 18px; font-size: 0.875rem;
    color: var(--gray-600); border-left: 3px solid transparent;
    transition: all 0.15s; cursor: pointer;
    text-decoration: none;
  }
  .sidebar-link:hover {
    background: var(--green-light); color: var(--green-dark);
    text-decoration: none;
  }
  .sidebar-link.active {
    background: var(--green-light); color: var(--green-dark);
    border-left-color: var(--green); font-weight: 600;
  }
  .sidebar-link .dot {
    width: 7px; height: 7px; border-radius: 50%;
    background: var(--gray-200); flex-shrink: 0;
  }
  .sidebar-link.active .dot, .sidebar-link:hover .dot { background: var(--green); }
  .sidebar-divider { height: 1px; background: var(--gray-100); margin: 6px 0; }

  /* ─── MAIN CONTENT ─── */
  .main-content {
    flex: 1; min-width: 0;
    padding: 32px 28px;
    max-width: 820px;
  }

  .breadcrumb {
    font-size: 0.78rem; color: var(--gray-400); margin-bottom: 18px;
    display: flex; align-items: center; gap: 6px;
  }
  .breadcrumb a { color: var(--green-dark); }
  .breadcrumb span { color: var(--gray-400); }

  .page-title {
    font-family: var(--font-heading);
    font-size: 2rem; font-weight: 700; color: var(--gray-900);
    margin-bottom: 6px; line-height: 1.25;
  }
  .page-subtitle {
    font-size: 1rem; color: var(--gray-600); margin-bottom: 28px;
  }

  /* Ad banner top */
  .ad-banner-top {
    background: #fff; border: 1px solid var(--gray-200);
    border-radius: var(--radius); padding: 14px 18px;
    margin-bottom: 28px;
    display: flex; align-items: center; justify-content: space-between;
    gap: 12px;
  }
  .ad-label {
    font-size: 0.65rem; text-transform: uppercase; letter-spacing: 1px;
    color: var(--gray-400); font-weight: 600;
  }
  .ad-banner-top .ad-inner {
    flex: 1; background: linear-gradient(90deg, #e8f5e9, #f1f8e9);
    border-radius: 4px; height: 70px;
    display: flex; align-items: center; justify-content: center;
    color: #388e3c; font-weight: 600; font-size: 0.85rem;
    border: 1px dashed #a5d6a7; letter-spacing: 0.3px;
  }

  /* Content sections */
  .content-section { margin-bottom: 32px; }
  .content-section h2 {
    font-family: var(--font-heading);
    font-size: 1.35rem; font-weight: 700; color: var(--gray-900);
    margin-bottom: 12px; padding-bottom: 8px;
    border-bottom: 2px solid var(--green-light);
  }
  .content-section h3 {
    font-size: 1rem; font-weight: 600; color: var(--gray-800);
    margin: 18px 0 8px;
  }
  .content-section p { margin-bottom: 12px; color: var(--gray-700); }
  .content-section ul, .content-section ol {
    margin: 0 0 12px 20px; color: var(--gray-700);
  }
  .content-section li { margin-bottom: 6px; }

  /* Code box */
  .code-box {
    background: var(--code-bg); border-radius: var(--radius);
    overflow: hidden; margin: 16px 0; box-shadow: var(--shadow-md);
  }
  .code-box-header {
    background: #21252b; padding: 8px 16px;
    display: flex; align-items: center; justify-content: space-between;
  }
  .code-box-lang {
    font-size: 0.72rem; text-transform: uppercase; letter-spacing: 1px;
    color: #6c7a89; font-weight: 600;
  }
  .code-box-actions { display: flex; gap: 8px; }
  .code-btn {
    font-size: 0.75rem; padding: 3px 10px; border-radius: 3px;
    cursor: pointer; border: none; font-family: var(--font-body);
    font-weight: 600; transition: opacity 0.15s;
  }
  .code-btn.run { background: var(--green); color: #fff; }
  .code-btn.copy { background: #3e4451; color: #abb2bf; }
  .code-btn:hover { opacity: 0.85; }
  .code-box pre {
    padding: 18px; overflow-x: auto;
    font-family: var(--font-code);
    font-size: 0.875rem; line-height: 1.65;
    color: var(--code-text);
  }
  .kw { color: #c678dd; }
  .fn { color: #61afef; }
  .str { color: #98c379; }
  .cm { color: #5c6370; font-style: italic; }
  .num { color: #d19a66; }

  /* Note / tip boxes */
  .note-box {
    border-left: 4px solid var(--green);
    background: var(--green-light); border-radius: 0 var(--radius) var(--radius) 0;
    padding: 14px 18px; margin: 16px 0;
    font-size: 0.9rem; color: var(--gray-700);
  }
  .note-box strong { color: var(--green-dark); }
  .warning-box {
    border-left: 4px solid #f59e0b;
    background: #fffbeb; border-radius: 0 var(--radius) var(--radius) 0;
    padding: 14px 18px; margin: 16px 0;
    font-size: 0.9rem; color: #78350f;
  }

  /* Nav buttons */
  .lesson-nav {
    display: flex; justify-content: space-between; align-items: center;
    margin-top: 40px; padding-top: 20px;
    border-top: 1px solid var(--gray-200);
    gap: 12px;
  }
  .nav-btn {
    display: flex; align-items: center; gap: 8px;
    background: var(--green); color: #fff;
    padding: 10px 20px; border-radius: var(--radius);
    font-size: 0.875rem; font-weight: 600;
    cursor: pointer; border: none; font-family: var(--font-body);
    transition: background 0.15s; text-decoration: none;
  }
  .nav-btn:hover { background: var(--green-dark); text-decoration: none; color: #fff; }
  .nav-btn.secondary { background: #fff; color: var(--gray-800); border: 1.5px solid var(--gray-200); }
  .nav-btn.secondary:hover { background: var(--gray-100); color: var(--gray-800); }
  .nav-btn-info { font-size: 0.78rem; color: var(--gray-400); text-align: center; }

  /* ─── RIGHT AD COLUMN ─── */
  .ad-column {
    width: var(--ad-width);
    min-width: var(--ad-width);
    padding: 28px 14px;
    flex-shrink: 0;
  }
  .ad-widget {
    background: #fff; border: 1px solid var(--gray-200);
    border-radius: var(--radius); overflow: hidden;
    margin-bottom: 20px;
    box-shadow: var(--shadow-sm);
  }
  .ad-widget-label {
    font-size: 0.62rem; text-transform: uppercase; letter-spacing: 1px;
    color: var(--gray-400); font-weight: 600;
    padding: 7px 10px; border-bottom: 1px solid var(--gray-100);
    background: var(--gray-50);
  }
  .ad-widget-body {
    padding: 14px 12px;
    min-height: 140px;
    display: flex; flex-direction: column;
    align-items: center; justify-content: center;
    gap: 8px; text-align: center;
  }
  .ad-placeholder {
    background: linear-gradient(135deg, #e8f5e9 0%, #f0fff4 100%);
    border: 1px dashed #a5d6a7; border-radius: 4px;
    width: 100%; padding: 20px 10px;
    font-size: 0.78rem; color: #388e3c; font-weight: 600;
    letter-spacing: 0.2px;
  }
  .ad-placeholder.tall { padding: 40px 10px; }
  .ad-placeholder.sky { padding: 60px 10px; }

  /* Promo card */
  .promo-card {
    background: linear-gradient(135deg, var(--green) 0%, #037d52 100%);
    border-radius: var(--radius); padding: 16px 14px;
    color: #fff; text-align: center; margin-bottom: 20px;
  }
  .promo-card h4 { font-size: 0.9rem; font-weight: 700; margin-bottom: 6px; }
  .promo-card p { font-size: 0.75rem; opacity: 0.9; margin-bottom: 12px; }
  .promo-card button {
    background: #fff; color: var(--green-dark);
    border: none; border-radius: 4px; padding: 7px 16px;
    font-size: 0.8rem; font-weight: 700; cursor: pointer;
  }

  /* Progress */
  .progress-card {
    background: #fff; border: 1px solid var(--gray-200);
    border-radius: var(--radius); padding: 14px; margin-bottom: 20px;
    box-shadow: var(--shadow-sm);
  }
  .progress-title { font-size: 0.8rem; font-weight: 600; color: var(--gray-800); margin-bottom: 10px; }
  .progress-bar-bg {
    background: var(--gray-100); border-radius: 10px; height: 8px; overflow: hidden;
  }
  .progress-bar-fill {
    height: 100%; border-radius: 10px;
    background: linear-gradient(90deg, var(--green), #37d08e);
    width: 34%;
  }
  .progress-meta { font-size: 0.72rem; color: var(--gray-400); margin-top: 6px; }

  /* Inline tag */
  code {
    font-family: var(--font-code); font-size: 0.85em;
    background: var(--gray-100); padding: 1px 6px; border-radius: 3px;
    color: #c0392b;
  }

  /* Table */
  .content-table {
    width: 100%; border-collapse: collapse;
    margin: 16px 0; font-size: 0.875rem;
  }
  .content-table th {
    background: var(--green); color: #fff;
    padding: 9px 14px; text-align: left; font-weight: 600;
    font-size: 0.82rem;
  }
  .content-table td {
    padding: 9px 14px; border-bottom: 1px solid var(--gray-200);
    color: var(--gray-700);
  }
  .content-table tr:nth-child(even) td { background: var(--gray-50); }
  .content-table tr:hover td { background: var(--green-light); }

  /* ─── FOOTER ─── */
  .site-footer {
    background: var(--gray-900); color: var(--gray-400);
    padding: 32px 28px; font-size: 0.82rem;
    display: flex; flex-wrap: wrap; gap: 24px;
    justify-content: space-between;
  }
  .footer-col h5 { color: #fff; font-size: 0.85rem; margin-bottom: 10px; }
  .footer-col a { color: var(--gray-400); display: block; margin-bottom: 6px; }
  .footer-col a:hover { color: var(--green); text-decoration: none; }
  .footer-bottom {
    background: #0a0f14; color: var(--gray-400);
    text-align: center; padding: 14px;
    font-size: 0.78rem; border-top: 1px solid #1f2937;
  }

  /* ─── MOBILE OVERLAY ─── */
  .sidebar-overlay {
    display: none; position: fixed; inset: 0; z-index: 900;
    background: rgba(0,0,0,0.45);
  }

  /* ─── RESPONSIVE ─── */
  @media (max-width: 1100px) {
    .ad-column { display: none; }
  }

  @media (max-width: 820px) {
    .sidebar {
      position: fixed; left: -280px; top: var(--header-h);
      height: calc(100vh - var(--header-h));
      z-index: 950; transition: left 0.25s ease;
      box-shadow: var(--shadow-md);
    }
    .sidebar.open { left: 0; }
    .sidebar-overlay.active { display: block; }
    .hamburger { display: flex; }
    .main-content { padding: 20px 16px; }
    .page-title { font-size: 1.55rem; }
    .topnav { padding: 0 14px; }
    .topnav-logo .brand-tagline { display: none; }
    .topnav-logo .brand-name { font-size: 1rem; }
    .topnav-logo .logo-icon { width: 32px; height: 32px; font-size: 0.95rem; }
    .btn-outline { padding: 6px 12px; font-size: 0.78rem; }
    .btn-solid { padding: 6px 12px; font-size: 0.78rem; }
  }

  @media (max-width: 480px) {
    .topnav { padding: 0 10px; }
    .topnav-logo .brand-name { font-size: 0.82rem; letter-spacing: -0.2px; }
    .topnav-logo .logo-icon { width: 28px; height: 28px; font-size: 0.8rem; border-radius: 7px; }
    .topnav-logo { gap: 7px; }
    .btn-outline { display: none; }
    .btn-solid { padding: 6px 11px; font-size: 0.76rem; }
    .lesson-nav { flex-direction: column; }
    .nav-btn { width: 100%; justify-content: center; }
  }