:root {
  --bg: #f5f6fb;
  --surface: #ffffff;
  --text: #1a1b2e;
  --muted: #5a5d77;
  --border: #e3e5ef;
  --primary: #5b4dff;
  --primary-dark: #3f33d6;
  --accent: #ff7a59;
  --success: #1f9d55;
  --success-bg: #e4f6ec;
  --danger: #d23c3c;
  --danger-bg: #fbe6e6;
  --code-bg: #0f1226;
  --code-text: #e6e7ef;
  --shadow-sm: 0 1px 2px rgba(20, 22, 50, 0.06);
  --shadow-md: 0 8px 24px rgba(20, 22, 50, 0.08);
  --radius: 12px;
  --radius-sm: 8px;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto,
    "Helvetica Neue", Arial, sans-serif;
  color: var(--text);
  background: var(--bg);
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}

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

.container {
  width: 100%;
  max-width: 1080px;
  margin: 0 auto;
  padding: 0 20px;
}

/* Header */
.site-header {
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 10;
}

.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  padding: 14px 20px;
}

.logo {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  font-weight: 700;
  font-size: 1.05rem;
  color: var(--text);
  text-decoration: none;
}

.logo-mark {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 32px;
  height: 32px;
  border-radius: 8px;
  background: linear-gradient(135deg, var(--primary) 0%, var(--accent) 100%);
  color: white;
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 0.85rem;
}

.site-nav {
  display: flex;
  flex-wrap: wrap;
  gap: 4px;
}

.site-nav a {
  color: var(--muted);
  text-decoration: none;
  padding: 8px 12px;
  border-radius: 8px;
  font-weight: 500;
  font-size: 0.95rem;
  transition: color 0.15s, background 0.15s;
}

.site-nav a:hover { color: var(--text); background: var(--bg); }
.site-nav a.active { color: var(--primary); background: rgba(91, 77, 255, 0.08); }

/* Hero */
.hero {
  background:
    radial-gradient(ellipse at top right, rgba(255, 122, 89, 0.12), transparent 60%),
    radial-gradient(ellipse at bottom left, rgba(91, 77, 255, 0.14), transparent 55%),
    var(--surface);
  border-bottom: 1px solid var(--border);
  padding: 60px 0 70px;
}

.hero-inner {
  display: grid;
  grid-template-columns: 1.15fr 1fr;
  gap: 48px;
  align-items: center;
}

.hero-copy h1 {
  font-size: clamp(2rem, 4vw, 3rem);
  line-height: 1.1;
  margin: 12px 0 16px;
  letter-spacing: -0.02em;
}

.eyebrow {
  text-transform: uppercase;
  letter-spacing: 0.08em;
  font-size: 0.78rem;
  font-weight: 700;
  color: var(--primary);
  margin: 0;
}

.lead {
  color: var(--muted);
  font-size: 1.1rem;
  max-width: 52ch;
  margin: 0 0 24px;
}

.hero-ctas {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
}

.hero-art {
  background: var(--code-bg);
  border-radius: var(--radius);
  padding: 20px 22px;
  box-shadow: var(--shadow-md);
  position: relative;
  overflow: hidden;
}

.hero-art::before {
  content: "";
  position: absolute;
  top: 12px;
  left: 16px;
  width: 48px;
  height: 10px;
  background:
    radial-gradient(circle at 5px 5px, #ff5f56 5px, transparent 5px),
    radial-gradient(circle at 21px 5px, #ffbd2e 5px, transparent 5px),
    radial-gradient(circle at 37px 5px, #27c93f 5px, transparent 5px);
}

.code-preview {
  margin: 24px 0 0;
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 0.92rem;
  color: var(--code-text);
  white-space: pre;
  overflow-x: auto;
}

/* Buttons */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  padding: 11px 20px;
  border-radius: 10px;
  font-weight: 600;
  font-size: 0.95rem;
  text-decoration: none;
  border: 1px solid transparent;
  cursor: pointer;
  transition: transform 0.1s, background 0.15s, border-color 0.15s;
  font-family: inherit;
}

.btn:active { transform: translateY(1px); }

.btn-primary {
  background: var(--primary);
  color: white;
}
.btn-primary:hover { background: var(--primary-dark); }

.btn-ghost {
  background: transparent;
  color: var(--text);
  border-color: var(--border);
}
.btn-ghost:hover { background: var(--surface); border-color: var(--muted); }

/* Sections */
.section { padding: 60px 0; }
.section h2 {
  font-size: 1.75rem;
  letter-spacing: -0.01em;
  margin: 0 0 8px;
}
.section-sub { color: var(--muted); margin: 0 0 28px; max-width: 60ch; }

/* Card grid */
.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
  gap: 18px;
}

.card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 22px;
  color: var(--text);
  text-decoration: none;
  display: flex;
  flex-direction: column;
  gap: 8px;
  box-shadow: var(--shadow-sm);
  transition: transform 0.15s, box-shadow 0.15s, border-color 0.15s;
}
.card:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-md);
  border-color: transparent;
}
.card h3 { margin: 4px 0 0; font-size: 1.15rem; }
.card p { margin: 0; color: var(--muted); font-size: 0.95rem; }
.card-link {
  margin-top: auto;
  color: var(--primary);
  font-weight: 600;
  font-size: 0.9rem;
  padding-top: 8px;
}
.card-icon {
  width: 38px;
  height: 38px;
  border-radius: 10px;
  background: var(--accent);
  color: white;
  font-weight: 700;
  display: inline-flex;
  align-items: center;
  justify-content: center;
}

/* Value grid */
.value-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 18px;
}
.value {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 22px;
  box-shadow: var(--shadow-sm);
}
.value h3 { margin-top: 0; font-size: 1.05rem; }
.value p { margin-bottom: 0; color: var(--muted); font-size: 0.95rem; }

.tip {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 28px;
  box-shadow: var(--shadow-sm);
  margin-bottom: 60px;
}

/* Page hero (inner pages) */
.page-hero {
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  padding: 48px 0 36px;
}
.page-hero h1 {
  font-size: clamp(1.8rem, 3.5vw, 2.6rem);
  margin: 8px 0 12px;
  letter-spacing: -0.02em;
}

/* Lesson layout */
.lesson-layout {
  display: grid;
  grid-template-columns: 220px 1fr;
  gap: 40px;
  padding: 40px 20px 64px;
  align-items: start;
}

.lesson-toc {
  position: sticky;
  top: 80px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 18px 20px;
  box-shadow: var(--shadow-sm);
}
.lesson-toc h4 {
  margin: 0 0 10px;
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--muted);
}
.lesson-toc ol {
  list-style: none;
  padding: 0;
  margin: 0;
  counter-reset: toc;
}
.lesson-toc li { counter-increment: toc; font-size: 0.93rem; margin-bottom: 2px; }
.lesson-toc a {
  display: block;
  padding: 6px 8px;
  color: var(--muted);
  text-decoration: none;
  border-radius: 6px;
}
.lesson-toc a:hover { background: var(--bg); color: var(--text); }

.lesson {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 36px 40px;
  box-shadow: var(--shadow-sm);
}

.lesson section { margin-bottom: 36px; }
.lesson section:last-of-type { margin-bottom: 0; }
.lesson h2 {
  font-size: 1.5rem;
  margin: 0 0 12px;
  letter-spacing: -0.01em;
  border-bottom: 2px solid var(--border);
  padding-bottom: 8px;
}
.lesson p { margin: 0 0 14px; }
.lesson code {
  background: #eef0fb;
  color: #2a2d4a;
  padding: 2px 6px;
  border-radius: 5px;
  font-size: 0.9em;
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
}

/* Code blocks */
.code {
  background: var(--code-bg);
  color: var(--code-text);
  padding: 18px 20px;
  border-radius: var(--radius-sm);
  overflow-x: auto;
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 0.92rem;
  line-height: 1.55;
  margin: 0 0 16px;
  white-space: pre;
}
.c-keyword { color: #c792ea; }
.c-string { color: #c3e88d; }
.c-num { color: #f78c6c; }
.c-comment { color: #8b92b0; font-style: italic; }
.c-op { color: #89ddff; }

/* Callouts & extras */
.callout {
  background: #fff7e8;
  border-left: 4px solid #ffbe0b;
  padding: 12px 16px;
  border-radius: var(--radius-sm);
  margin: 12px 0 18px;
  font-size: 0.97rem;
}

.try-box {
  background: #eef6ff;
  border: 1px solid #c8dcf3;
  border-radius: var(--radius-sm);
  padding: 16px 18px;
  margin: 14px 0 18px;
}
.try-box h4 { margin: 0 0 6px; color: #1d4e89; font-size: 0.95rem; }

.project {
  background: linear-gradient(135deg, #f0edff 0%, #fff3ec 100%);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 22px 24px;
}
.project h2 { border-bottom: none; padding-bottom: 0; }

.tips, .steps {
  padding-left: 22px;
  margin: 8px 0 16px;
}
.tips li, .steps li { margin-bottom: 6px; }

.data-table {
  width: 100%;
  border-collapse: collapse;
  margin: 8px 0 18px;
  font-size: 0.95rem;
}
.data-table th, .data-table td {
  border: 1px solid var(--border);
  padding: 8px 12px;
  text-align: left;
}
.data-table thead { background: var(--bg); }

.glossary { margin: 0; }
.glossary dt { font-weight: 700; margin-top: 10px; }
.glossary dd { margin: 2px 0 8px 0; color: var(--muted); }

.preview {
  border: 1px dashed var(--border);
  border-radius: var(--radius-sm);
  padding: 14px 16px;
  margin: 0 0 18px;
  background: #fafbff;
}
.preview-label {
  margin: 0 0 8px;
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--muted);
}
.preview-body { margin: 0; }
.mono { font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace; }

.lesson-nav {
  display: flex;
  justify-content: space-between;
  gap: 12px;
  margin-top: 32px;
  flex-wrap: wrap;
}

/* Quiz */
.quiz-tabs {
  display: flex;
  gap: 6px;
  margin-bottom: 24px;
  background: var(--surface);
  padding: 6px;
  border: 1px solid var(--border);
  border-radius: 12px;
  width: fit-content;
  flex-wrap: wrap;
}
.quiz-tab {
  background: transparent;
  border: none;
  padding: 10px 18px;
  font-weight: 600;
  color: var(--muted);
  cursor: pointer;
  border-radius: 8px;
  font-family: inherit;
  font-size: 0.95rem;
}
.quiz-tab:hover { color: var(--text); }
.quiz-tab.active { background: var(--primary); color: white; }

.quiz {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 28px 32px;
  box-shadow: var(--shadow-sm);
}

.quiz-q {
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 18px 20px;
  margin: 0 0 16px;
}
.quiz-q legend {
  font-weight: 600;
  padding: 0 6px;
  font-size: 1rem;
}
.q-num { color: var(--primary); margin-right: 4px; }

.quiz-option {
  display: flex;
  align-items: flex-start;
  gap: 10px;
  padding: 10px 12px;
  border-radius: 8px;
  cursor: pointer;
  border: 1px solid transparent;
  transition: background 0.12s, border-color 0.12s;
  font-size: 0.97rem;
}
.quiz-option:hover { background: var(--bg); }
.quiz-option input { margin-top: 3px; accent-color: var(--primary); }
.quiz-option.correct {
  background: var(--success-bg);
  border-color: var(--success);
}
.quiz-option.incorrect {
  background: var(--danger-bg);
  border-color: var(--danger);
}

.quiz-feedback {
  margin: 10px 6px 0;
  font-size: 0.92rem;
  min-height: 0;
}
.quiz-feedback.correct { color: var(--success); }
.quiz-feedback.incorrect { color: var(--danger); }
.quiz-feedback:empty { display: none; }

.quiz-actions {
  display: flex;
  gap: 10px;
  margin-top: 6px;
  flex-wrap: wrap;
}

.quiz-result {
  margin-top: 18px;
  padding: 18px 22px;
  background: linear-gradient(135deg, #eef5ff 0%, #fef3ec 100%);
  border: 1px solid var(--border);
  border-radius: var(--radius);
}
.quiz-result h3 { margin: 0 0 6px; font-size: 1.2rem; }
.quiz-result p { margin: 0; color: var(--muted); }

/* Footer */
.site-footer {
  border-top: 1px solid var(--border);
  background: var(--surface);
  padding: 24px 0;
  margin-top: 40px;
  color: var(--muted);
  font-size: 0.92rem;
  text-align: center;
}

/* Responsive */
@media (max-width: 860px) {
  .hero { padding: 44px 0 54px; }
  .hero-inner { grid-template-columns: 1fr; gap: 32px; }
  .lesson-layout { grid-template-columns: 1fr; padding: 28px 16px 48px; gap: 24px; }
  .lesson-toc { position: static; }
  .lesson { padding: 28px 22px; }
  .quiz { padding: 22px 20px; }
}

@media (max-width: 540px) {
  .header-inner { flex-direction: column; align-items: flex-start; }
  .site-nav { width: 100%; justify-content: flex-start; }
  .hero-copy h1 { font-size: 1.9rem; }
  .lesson h2 { font-size: 1.25rem; }
}