/* ===========================================================
   Pólya – Phương Pháp Giải Toán | Stylesheet chung
   =========================================================== */
:root {
  --primary: #1e40af;
  --primary-light: #3b82f6;
  --primary-dark: #1e3a8a;
  --accent: #f59e0b;
  --accent-dark: #d97706;
  --bg: #f6f8fc;
  --card: #ffffff;
  --text: #1e293b;
  --text-light: #64748b;
  --border: #e2e8f0;
  --success: #10b981;
  --danger: #ef4444;
  --highlight: #fef3c7;
  --code-bg: #f1f5f9;
  --shadow-sm: 0 1px 2px rgba(15,23,42,.06);
  --shadow: 0 4px 16px rgba(15,23,42,.08);
  --shadow-lg: 0 12px 40px rgba(15,23,42,.12);
  --radius: 14px;
  --sidebar-w: 280px;
  --font: 'Be Vietnam Pro', 'Segoe UI', 'Helvetica Neue', Arial, 'Noto Sans', sans-serif;
  --serif: 'Lora', Georgia, 'Times New Roman', serif;
}

[data-theme="dark"] {
  --primary: #60a5fa;
  --primary-light: #93c5fd;
  --primary-dark: #3b82f6;
  --accent: #fbbf24;
  --bg: #0f172a;
  --card: #1e293b;
  --text: #e2e8f0;
  --text-light: #94a3b8;
  --border: #334155;
  --highlight: #422006;
  --code-bg: #0b1220;
  --shadow: 0 4px 16px rgba(0,0,0,.4);
  --shadow-lg: 0 12px 40px rgba(0,0,0,.5);
}

* { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; scroll-padding-top: 80px; }
body {
  font-family: var(--font);
  line-height: 1.75;
  color: var(--text);
  background: var(--bg);
  transition: background .3s, color .3s;
}

/* ---------- Topbar ---------- */
.topbar {
  position: sticky; top: 0; z-index: 100;
  background: var(--card);
  border-bottom: 1px solid var(--border);
  display: flex; align-items: center; gap: 16px;
  padding: 12px 22px;
  box-shadow: var(--shadow-sm);
}
.topbar .brand {
  display: flex; align-items: center; gap: 12px;
  font-weight: 700; font-size: 1.05rem; color: var(--text);
  text-decoration: none;
}
.topbar .brand .logo {
  width: 38px; height: 38px; border-radius: 10px; flex-shrink: 0;
  background: linear-gradient(135deg, var(--primary), var(--primary-light));
  display: grid; place-items: center; color: #fff; font-weight: 800;
  font-family: var(--serif); font-size: 1.2rem;
}
.topbar .brand small { display: block; font-weight: 400; font-size: .72rem; color: var(--text-light); }
.topbar .spacer { flex: 1; }
.topbar .icon-btn {
  background: transparent; border: 1px solid var(--border); color: var(--text);
  width: 42px; height: 42px; border-radius: 10px; cursor: pointer;
  display: grid; place-items: center; font-size: 1.05rem; transition: .2s;
}
.topbar .icon-btn:hover { background: var(--bg); border-color: var(--primary-light); color: var(--primary); }
.menu-toggle { display: none; }

/* ---------- Layout ---------- */
.layout { display: flex; min-height: calc(100vh - 64px); }

/* ---------- Sidebar ---------- */
.sidebar {
  width: var(--sidebar-w); flex-shrink: 0;
  background: var(--card);
  border-right: 1px solid var(--border);
  padding: 22px 14px 60px;
  position: sticky; top: 64px; align-self: flex-start;
  height: calc(100vh - 64px); overflow-y: auto;
}
.sidebar h4 {
  font-size: .72rem; text-transform: uppercase; letter-spacing: .08em;
  color: var(--text-light); margin: 18px 12px 8px; font-weight: 700;
}
.sidebar a {
  display: flex; align-items: center; gap: 10px;
  padding: 9px 12px; border-radius: 9px; margin-bottom: 2px;
  color: var(--text); text-decoration: none; font-size: .92rem;
  transition: .15s; border-left: 3px solid transparent;
}
.sidebar a i { width: 20px; text-align: center; color: var(--text-light); font-size: .9rem; }
.sidebar a:hover { background: var(--bg); color: var(--primary); }
.sidebar a:hover i { color: var(--primary); }
.sidebar a.active {
  background: linear-gradient(90deg, rgba(59,130,246,.12), transparent);
  color: var(--primary); font-weight: 600; border-left-color: var(--primary);
}
.sidebar a.active i { color: var(--primary); }

/* ---------- Main content ---------- */
.main { flex: 1; min-width: 0; padding: 34px clamp(18px, 5vw, 64px) 90px; max-width: 980px; margin: 0 auto; }
.breadcrumb { font-size: .85rem; color: var(--text-light); margin-bottom: 18px; }
.breadcrumb a { color: var(--primary); text-decoration: none; }
.breadcrumb a:hover { text-decoration: underline; }

/* ---------- Typography ---------- */
.main h1 { font-family: var(--serif); font-size: clamp(1.7rem, 4vw, 2.5rem); color: var(--text); line-height: 1.25; margin-bottom: 10px; }
.main h2 { font-family: var(--serif); font-size: 1.55rem; margin: 42px 0 14px; color: var(--text); padding-bottom: 8px; border-bottom: 2px solid var(--border); }
.main h3 { font-size: 1.2rem; margin: 28px 0 10px; color: var(--primary); }
.main h4 { font-size: 1.02rem; margin: 18px 0 6px; }
.main p { margin-bottom: 14px; }
.main ul, .main ol { margin: 0 0 16px 22px; }
.main li { margin-bottom: 7px; }
.lead { font-size: 1.12rem; color: var(--text-light); margin-bottom: 24px; }
.main a.inline { color: var(--primary); }

/* ---------- Cards / boxes ---------- */
.card {
  background: var(--card); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 24px; box-shadow: var(--shadow-sm); margin-bottom: 20px;
}
.card-grid { display: grid; gap: 18px; grid-template-columns: repeat(auto-fit, minmax(240px,1fr)); margin: 22px 0; }
.phase-card {
  background: var(--card); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 22px; box-shadow: var(--shadow-sm); transition: .25s; text-decoration: none; color: var(--text);
  display: block; position: relative; overflow: hidden;
}
.phase-card:hover { transform: translateY(-4px); box-shadow: var(--shadow-lg); border-color: var(--primary-light); }
.phase-card .num {
  width: 46px; height: 46px; border-radius: 12px; display: grid; place-items: center;
  font-family: var(--serif); font-weight: 800; font-size: 1.4rem; color: #fff; margin-bottom: 14px;
  background: linear-gradient(135deg, var(--primary), var(--primary-light));
}
.phase-card h3 { margin: 0 0 6px; color: var(--text); font-size: 1.12rem; }
.phase-card p { color: var(--text-light); font-size: .9rem; margin: 0; }

/* Callouts */
.callout { border-radius: 12px; padding: 16px 20px; margin: 20px 0; border-left: 5px solid; }
.callout .ttl { font-weight: 700; display: flex; align-items: center; gap: 8px; margin-bottom: 6px; }
.callout p:last-child { margin-bottom: 0; }
.callout.tip { background: rgba(16,185,129,.08); border-color: var(--success); }
.callout.tip .ttl { color: var(--success); }
.callout.quote { background: var(--highlight); border-color: var(--accent); font-family: var(--serif); }
.callout.quote .ttl { color: var(--accent-dark); }
.callout.note { background: rgba(59,130,246,.08); border-color: var(--primary-light); }
.callout.note .ttl { color: var(--primary); }
.callout.warn { background: rgba(239,68,68,.07); border-color: var(--danger); }
.callout.warn .ttl { color: var(--danger); }

/* Question list */
.q-list { list-style: none; margin-left: 0 !important; }
.q-list li {
  background: var(--card); border: 1px solid var(--border); border-radius: 10px;
  padding: 12px 16px 12px 44px; margin-bottom: 10px; position: relative; box-shadow: var(--shadow-sm);
}
.q-list li::before {
  content: "\f059"; font-family: "Font Awesome 6 Free"; font-weight: 900;
  position: absolute; left: 14px; top: 13px; color: var(--accent);
}

/* Step blocks for examples */
.step { display: flex; gap: 16px; margin-bottom: 18px; }
.step .step-num {
  flex-shrink: 0; width: 34px; height: 34px; border-radius: 50%;
  background: var(--primary); color: #fff; display: grid; place-items: center; font-weight: 700;
}
.step .step-body { flex: 1; }
.step .step-body h4 { margin-top: 4px; }

/* Problem statement box */
.problem-box {
  background: linear-gradient(135deg, rgba(30,64,175,.06), rgba(59,130,246,.04));
  border: 1px solid var(--primary-light); border-radius: var(--radius);
  padding: 20px 24px; margin: 18px 0;
}
.problem-box .label {
  display: inline-block; background: var(--primary); color: #fff; font-size: .72rem;
  font-weight: 700; padding: 3px 12px; border-radius: 20px; letter-spacing: .05em; margin-bottom: 10px; text-transform: uppercase;
}
.problem-box p:last-child { margin-bottom: 0; }

/* Tables */
.tbl-wrap { overflow-x: auto; margin: 18px 0; }
table.styled { width: 100%; border-collapse: collapse; font-size: .92rem; background: var(--card); border-radius: 10px; overflow: hidden; box-shadow: var(--shadow-sm); }
table.styled th, table.styled td { padding: 12px 14px; text-align: left; border-bottom: 1px solid var(--border); vertical-align: top; }
table.styled th { background: var(--primary); color: #fff; font-weight: 600; }
table.styled tr:nth-child(even) td { background: var(--bg); }

/* Example index cards */
.ex-list { display: grid; gap: 16px; }
.ex-card {
  display: flex; gap: 16px; align-items: flex-start; text-decoration: none; color: var(--text);
  background: var(--card); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 18px 20px; box-shadow: var(--shadow-sm); transition: .2s;
}
.ex-card:hover { border-color: var(--primary-light); box-shadow: var(--shadow); transform: translateX(4px); }
.ex-card .ico {
  flex-shrink: 0; width: 48px; height: 48px; border-radius: 12px; display: grid; place-items: center;
  font-size: 1.3rem; color: #fff; background: linear-gradient(135deg, var(--accent), var(--accent-dark));
}
.ex-card h3 { margin: 0 0 4px; font-size: 1.08rem; color: var(--text); }
.ex-card p { margin: 0; color: var(--text-light); font-size: .88rem; }
.ex-card .tag { font-size: .72rem; color: var(--primary); font-weight: 600; }

/* Dictionary */
.dict-search { width: 100%; padding: 14px 18px 14px 46px; border-radius: 12px; border: 1px solid var(--border);
  background: var(--card) url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='18' height='18' fill='%2364748b' viewBox='0 0 16 16'%3E%3Cpath d='M11.742 10.344a6.5 6.5 0 1 0-1.397 1.398h-.001q.044.06.098.115l3 3a1 1 0 0 0 1.415-1.414l-3-3a1 1 0 0 0-.115-.098M12 6.5a5.5 5.5 0 1 1-11 0 5.5 5.5 0 0 1 11 0'/%3E%3C/svg%3E") no-repeat 16px center;
  font-size: 1rem; color: var(--text); margin-bottom: 22px; font-family: var(--font); }
.dict-search:focus { outline: none; border-color: var(--primary); box-shadow: 0 0 0 3px rgba(59,130,246,.15); }
.dict-entry { background: var(--card); border: 1px solid var(--border); border-radius: var(--radius); padding: 18px 22px; margin-bottom: 14px; box-shadow: var(--shadow-sm); }
.dict-entry h3 { margin: 0 0 8px; color: var(--primary); display: flex; align-items: center; gap: 10px; }
.dict-entry h3 .vi { color: var(--text-light); font-size: .85rem; font-weight: 400; }
.dict-entry p:last-child { margin-bottom: 0; }
.dict-letters { display: flex; flex-wrap: wrap; gap: 6px; margin-bottom: 22px; }
.dict-letters button {
  width: 36px; height: 36px; border-radius: 8px; border: 1px solid var(--border); background: var(--card);
  color: var(--text); cursor: pointer; font-weight: 600; transition: .15s;
}
.dict-letters button:hover, .dict-letters button.active { background: var(--primary); color: #fff; border-color: var(--primary); }
.no-result { text-align: center; color: var(--text-light); padding: 40px; display: none; }

/* Hero */
.hero { text-align: center; padding: 30px 0 10px; }
.hero .badge { display: inline-block; background: var(--highlight); color: var(--accent-dark); padding: 6px 16px; border-radius: 30px; font-size: .82rem; font-weight: 600; margin-bottom: 18px; }
.hero h1 { font-size: clamp(2rem, 5vw, 3rem); }
.hero .author { font-size: 1.15rem; color: var(--text-light); margin-top: 6px; }

/* Pills / chips */
.chips { display: flex; flex-wrap: wrap; gap: 8px; margin: 14px 0; }
.chip { background: var(--code-bg); color: var(--text-light); padding: 5px 14px; border-radius: 20px; font-size: .82rem; border: 1px solid var(--border); }

/* progress nav between pages */
.page-nav { display: flex; justify-content: space-between; gap: 14px; margin-top: 50px; padding-top: 24px; border-top: 1px solid var(--border); flex-wrap: wrap; }
.page-nav a { flex: 1; min-width: 200px; text-decoration: none; padding: 14px 18px; border: 1px solid var(--border); border-radius: 12px; color: var(--text); transition: .2s; background: var(--card); }
.page-nav a:hover { border-color: var(--primary-light); box-shadow: var(--shadow); }
.page-nav a span { display: block; font-size: .75rem; color: var(--text-light); }
.page-nav a strong { color: var(--primary); }
.page-nav a.next { text-align: right; }

footer.site { background: var(--card); border-top: 1px solid var(--border); padding: 26px; text-align: center; color: var(--text-light); font-size: .88rem; }

/* math display nicer */
mjx-container { overflow-x: auto; overflow-y: hidden; max-width: 100%; }

/* ---------- Responsive ---------- */
.backdrop { display: none; }
@media (max-width: 900px) {
  .menu-toggle { display: grid; }
  .sidebar {
    position: fixed; top: 0; left: 0; height: 100vh; z-index: 200;
    transform: translateX(-100%); transition: transform .3s; box-shadow: var(--shadow-lg);
    padding-top: 70px;
  }
  .sidebar.open { transform: translateX(0); }
  .backdrop.show { display: block; position: fixed; inset: 0; background: rgba(0,0,0,.45); z-index: 150; }
  .main { padding: 24px 18px 70px; }
}