:root {
  --bg: #0d1117;
  --bg-soft: #111722;
  --bg-card: #161c28;
  --bg-card-hover: #1c2434;
  --border: #232c3d;
  --text: #e6edf3;
  --text-dim: #9aa7b8;
  --text-mute: #6e7b8e;
  --primary: #4f8cff;
  --primary-strong: #6ea2ff;
  --primary-bg: rgba(79, 140, 255, 0.12);
  --green: #3fb950;
  --green-bg: rgba(63, 185, 80, 0.12);
  --orange: #f0883e;
  --orange-bg: rgba(240, 136, 62, 0.12);
  --red: #f85149;
  --red-bg: rgba(248, 81, 73, 0.12);
  --yellow: #d29922;
  --purple: #a371f7;
  --code-bg: #0b1020;
  --radius: 10px;
  --radius-sm: 7px;
  --shadow: 0 6px 24px rgba(0, 0, 0, 0.35);
  --font: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif;
  --mono: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--font);
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  font-size: 16px;
}
.container { max-width: 1160px; margin: 0 auto; padding: 0 20px; }
a { color: var(--primary-strong); text-decoration: none; }
a:hover { text-decoration: underline; }
button { font-family: inherit; cursor: pointer; }
code, pre { font-family: var(--mono); }

::selection { background: var(--primary); color: #fff; }
::-webkit-scrollbar { width: 10px; height: 10px; }
::-webkit-scrollbar-track { background: var(--bg); }
::-webkit-scrollbar-thumb { background: var(--border); border-radius: 5px; }

/* ---------- Header ---------- */
.site-header {
  position: sticky;
  top: 0;
  z-index: 50;
  background: rgba(13, 17, 23, 0.88);
  backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--border);
}
.header-inner { display: flex; align-items: center; gap: 24px; height: 62px; }
.brand { display: flex; align-items: center; gap: 10px; font-size: 18px; color: var(--text); }
.brand:hover { text-decoration: none; }
.brand strong { color: var(--primary-strong); }
.brand-logo {
  background: var(--primary);
  color: #fff;
  border-radius: 8px;
  padding: 3px 7px;
  font-family: var(--mono);
  font-size: 14px;
  font-weight: 700;
}
.main-nav { display: flex; gap: 4px; margin-left: 8px; }
.main-nav a {
  color: var(--text-dim);
  padding: 7px 12px;
  border-radius: var(--radius-sm);
  font-size: 14.5px;
  transition: background 0.15s, color 0.15s;
}
.main-nav a:hover { color: var(--text); background: var(--bg-card); text-decoration: none; }
.main-nav a.active { color: var(--text); background: var(--primary-bg); }
.header-actions { margin-left: auto; display: flex; align-items: center; gap: 12px; }
.nav-toggle { display: none; background: none; border: 1px solid var(--border); color: var(--text); border-radius: 7px; padding: 6px 10px; font-size: 16px; }

/* ---------- Buttons ---------- */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  background: var(--primary-bg);
  color: var(--primary-strong);
  border: 1px solid var(--primary);
  border-radius: var(--radius-sm);
  padding: 9px 18px;
  font-size: 15px;
  font-weight: 600;
  transition: all 0.15s;
  text-decoration: none;
}
.btn:hover { background: var(--primary); color: #fff; text-decoration: none; }
.btn.primary { background: var(--primary); color: #fff; }
.btn.primary:hover { background: var(--primary-strong); }
.btn.ghost { background: transparent; border-color: var(--border); color: var(--text-dim); }
.btn.ghost:hover { color: var(--text); border-color: var(--text-mute); }
.btn.green { background: var(--green); color: #fff; border-color: var(--green); }
.btn.green:hover { background: #46d25a; }
.btn.sm { padding: 6px 12px; font-size: 13px; }
.btn.block { width: 100%; justify-content: center; }
.btn:disabled { opacity: 0.5; cursor: not-allowed; }

/* ---------- Hero ---------- */
.hero {
  padding: 80px 0 60px;
  background:
    radial-gradient(600px 300px at 20% 10%, rgba(79, 140, 255, 0.12), transparent),
    radial-gradient(600px 300px at 80% 20%, rgba(163, 113, 247, 0.1), transparent);
  border-bottom: 1px solid var(--border);
  text-align: center;
}
.hero h1 { font-size: 44px; line-height: 1.2; margin: 0 0 14px; }
.hero h1 span { color: var(--primary-strong); }
.hero p.lead { font-size: 19px; color: var(--text-dim); max-width: 680px; margin: 0 auto 28px; }
.hero-cta { display: flex; gap: 12px; justify-content: center; flex-wrap: wrap; }
.hero-stats {
  display: flex; gap: 40px; justify-content: center; margin-top: 44px; flex-wrap: wrap;
}
.hero-stats .stat b { display: block; font-size: 28px; color: var(--primary-strong); }
.hero-stats .stat span { color: var(--text-mute); font-size: 14px; }

/* ---------- Sections ---------- */
.section { padding: 56px 0; }
.section-title { font-size: 28px; margin: 0 0 6px; }
.section-sub { color: var(--text-dim); margin: 0 0 28px; }

.stage-block { margin-bottom: 40px; }
.stage-head { display: flex; align-items: center; gap: 12px; margin-bottom: 16px; }
.stage-badge {
  width: 38px; height: 38px; border-radius: 10px;
  display: flex; align-items: center; justify-content: center;
  font-weight: 700; font-size: 18px; color: #fff; flex-shrink: 0;
}
.stage-head h2 { margin: 0; font-size: 22px; }
.stage-head span.desc { color: var(--text-mute); font-size: 14px; }

.chapter-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(330px, 1fr)); gap: 16px; }
.chapter-card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 20px;
  transition: transform 0.15s, border-color 0.15s, background 0.15s;
  display: block;
  color: var(--text);
}
.chapter-card:hover { transform: translateY(-3px); border-color: var(--primary); text-decoration: none; background: var(--bg-card-hover); }
.chapter-card .ch-tag { font-size: 12px; color: var(--primary-strong); font-family: var(--mono); }
.chapter-card h3 { margin: 8px 0 6px; font-size: 18px; }
.chapter-card p { color: var(--text-dim); font-size: 14.5px; margin: 0; }
.chapter-card .ch-lessons { color: var(--text-mute); font-size: 13px; margin-top: 12px; }
.chapter-card .ch-lessons b { color: var(--text-dim); }

/* ---------- Features ---------- */
.feature-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(250px, 1fr)); gap: 16px; }
.feature {
  background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 22px;
}
.feature .ico { font-size: 26px; }
.feature h3 { margin: 10px 0 6px; font-size: 17px; }
.feature p { margin: 0; color: var(--text-dim); font-size: 14.5px; }

/* ---------- Auth ---------- */
.auth-wrap { max-width: 420px; margin: 60px auto; }
.auth-card { background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius); padding: 30px; }
.auth-card h1 { margin: 0 0 4px; font-size: 24px; }
.auth-card .sub { color: var(--text-mute); margin: 0 0 22px; font-size: 14px; }
.form-group { margin-bottom: 16px; }
.form-group label { display: block; font-size: 14px; color: var(--text-dim); margin-bottom: 6px; }
.form-group input, .form-group textarea, .form-group select {
  width: 100%; background: var(--bg); border: 1px solid var(--border); border-radius: var(--radius-sm);
  color: var(--text); padding: 10px 12px; font-size: 15px; font-family: inherit;
}
.form-group input:focus, .form-group textarea:focus { outline: none; border-color: var(--primary); }
.auth-switch { margin-top: 16px; text-align: center; color: var(--text-mute); font-size: 14px; }
.form-error { color: var(--red); font-size: 14px; margin: 0 0 12px; }

/* ---------- Learning layout ---------- */
.learn-layout { display: grid; grid-template-columns: 300px 1fr; gap: 24px; align-items: start; }
.sidebar {
  position: sticky; top: 82px;
  background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius);
  max-height: calc(100vh - 100px); overflow-y: auto;
}
.sidebar .side-head { padding: 16px; border-bottom: 1px solid var(--border); }
.sidebar .side-head h3 { margin: 0; font-size: 16px; }
.sidebar .side-head .percent { color: var(--text-mute); font-size: 13px; }
.progress-bar { height: 6px; background: var(--bg); border-radius: 4px; overflow: hidden; margin-top: 8px; }
.progress-bar > div { height: 100%; background: linear-gradient(90deg, var(--primary), var(--green)); transition: width 0.3s; }
.side-tree { padding: 10px; }
.tree-chapter { margin-bottom: 6px; }
.tree-chapter > .tc-head {
  display: flex; align-items: center; gap: 8px; padding: 8px 10px; border-radius: var(--radius-sm);
  color: var(--text); font-weight: 600; font-size: 14.5px; cursor: pointer; user-select: none;
}
.tree-chapter > .tc-head:hover { background: var(--bg-card-hover); }
.tree-chapter > .tc-head .tc-done { color: var(--green); font-size: 13px; margin-left: auto; font-weight: 400; }
.tree-chapter > .tc-head .chev { margin-left: auto; color: var(--text-mute); transition: transform 0.15s; font-size: 12px; }
.tree-chapter.open > .tc-head .chev { transform: rotate(90deg); }
.tc-lessons { display: none; padding-left: 12px; }
.tree-chapter.open > .tc-lessons { display: block; }
.tc-lessons a {
  display: flex; align-items: center; gap: 8px; padding: 7px 10px; border-radius: var(--radius-sm);
  color: var(--text-dim); font-size: 14px; border-left: 2px solid transparent;
}
.tc-lessons a:hover { color: var(--text); background: var(--bg-card-hover); text-decoration: none; }
.tc-lessons a.active { color: var(--primary-strong); border-left-color: var(--primary); background: var(--primary-bg); }
.tc-lessons a .mark { width: 16px; color: var(--text-mute); font-size: 12px; flex-shrink: 0; }
.tc-lessons a.done .mark { color: var(--green); }

/* ---------- Lesson content ---------- */
.lesson-main { min-width: 0; }
.lesson-head { border-bottom: 1px solid var(--border); padding-bottom: 18px; margin-bottom: 24px; }
.lesson-head .crumb { color: var(--text-mute); font-size: 13px; margin-bottom: 6px; }
.lesson-head .crumb a { color: var(--text-mute); }
.lesson-head h1 { margin: 0; font-size: 30px; }
.lesson-head .lesson-meta { color: var(--text-mute); font-size: 13px; margin-top: 8px; }

.prose { font-size: 16px; }
.prose h2 { font-size: 22px; margin: 34px 0 12px; padding-bottom: 8px; border-bottom: 1px solid var(--border); }
.prose h3 { font-size: 18px; margin: 26px 0 8px; }
.prose p { margin: 0 0 16px; }
.prose ul, .prose ol { padding-left: 24px; margin: 0 0 16px; }
.prose li { margin-bottom: 6px; }
.prose strong { color: var(--text); }

.callout { border-radius: var(--radius-sm); padding: 14px 16px; margin: 0 0 18px; font-size: 15px; border-left: 4px solid; }
.callout.tip { background: var(--green-bg); border-color: var(--green); color: #c7ecc9; }
.callout.warn { background: var(--orange-bg); border-color: var(--orange); color: #f3d6bb; }
.callout.info { background: var(--primary-bg); border-color: var(--primary); color: #cfe0ff; }
.callout b { color: inherit; }

.code-block { margin: 0 0 18px; border-radius: var(--radius); overflow: hidden; border: 1px solid var(--border); background: var(--code-bg); }
.code-head {
  display: flex; align-items: center; justify-content: space-between;
  background: var(--bg-card); padding: 8px 14px; border-bottom: 1px solid var(--border);
  font-size: 12.5px; color: var(--text-mute); font-family: var(--mono);
}
.code-head .actions { display: flex; gap: 8px; align-items: center; }
.code-head button {
  background: var(--bg); border: 1px solid var(--border); color: var(--text-dim);
  border-radius: 5px; padding: 3px 10px; font-size: 12px; font-family: inherit;
}
.code-head button:hover { color: var(--text); border-color: var(--text-mute); }
.code-block pre { margin: 0; padding: 14px 16px; overflow-x: auto; font-size: 14px; line-height: 1.55; }
.code-output { border-top: 1px solid var(--border); background: #05070d; }
.code-output pre { margin: 0; padding: 12px 16px; font-size: 13.5px; color: #7ee787; white-space: pre-wrap; word-break: break-word; }
.code-output pre.error { color: var(--red); }
.code-output .run-status { padding: 8px 16px; color: var(--text-mute); font-size: 12.5px; }
.code-output pre.loading { color: var(--text-mute); }

/* ---------- Quiz ---------- */
.quiz-wrap { margin-top: 40px; border-top: 1px solid var(--border); padding-top: 8px; }
.quiz-question { background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius); padding: 20px; margin-bottom: 16px; }
.quiz-question .q-title { font-weight: 600; margin-bottom: 12px; }
.quiz-question .q-title .q-num { color: var(--primary-strong); font-family: var(--mono); margin-right: 6px; }
.q-options label { display: flex; gap: 10px; align-items: flex-start; padding: 9px 12px; border: 1px solid var(--border); border-radius: var(--radius-sm); margin-bottom: 8px; cursor: pointer; font-size: 14.5px; transition: all 0.15s; }
.q-options label:hover { border-color: var(--primary); }
.q-options input { margin-top: 4px; accent-color: var(--primary); }
.q-options label.selected { border-color: var(--primary); background: var(--primary-bg); }
.q-options label.disabled { cursor: default; opacity: 0.75; }
.q-options label.correct { border-color: var(--green); background: var(--green-bg); }
.q-options label.wrong { border-color: var(--red); background: var(--red-bg); }
.q-explain { margin-top: 10px; padding: 10px 12px; border-radius: var(--radius-sm); background: var(--bg); font-size: 14px; display: none; }
.q-explain.show { display: block; }
.q-explain b { color: var(--green); }
.quiz-result { background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius); padding: 20px; margin-bottom: 16px; font-size: 15px; }
.quiz-result .big { font-size: 22px; font-weight: 700; }

/* ---------- Exercises ---------- */
.ex-nav { display: flex; flex-wrap: wrap; gap: 8px; margin-bottom: 24px; }
.ex-nav a { padding: 6px 14px; border-radius: 20px; border: 1px solid var(--border); color: var(--text-dim); font-size: 14px; }
.ex-nav a.active { background: var(--primary-bg); border-color: var(--primary); color: var(--primary-strong); }
.exercise-card { background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius); padding: 22px; margin-bottom: 18px; }
.exercise-card h2 { margin: 0 0 4px; font-size: 18px; }
.exercise-card .ex-id { color: var(--primary-strong); font-family: var(--mono); font-size: 13px; }
.exercise-card .ex-tags { margin: 8px 0 0; color: var(--text-mute); font-size: 13px; }
.solution { margin-top: 14px; border-top: 1px dashed var(--border); padding-top: 14px; }
.solution[hidden] { display: none; }

/* ---------- Playground ---------- */
.play-layout { display: grid; grid-template-columns: 1fr 1fr; gap: 20px; min-height: 70vh; }
.play-left, .play-right { display: flex; flex-direction: column; gap: 12px; }
.play-toolbar { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; }
.play-toolbar select { background: var(--bg-card); color: var(--text); border: 1px solid var(--border); border-radius: var(--radius-sm); padding: 9px 12px; font-size: 14px; }
.play-toolbar .spacer { flex: 1; }
#play-code {
  flex: 1; min-height: 400px; width: 100%;
  background: var(--code-bg); color: var(--text); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 16px; font-family: var(--mono); font-size: 14px; line-height: 1.55; resize: vertical;
}
#play-code:focus { outline: none; border-color: var(--primary); }
.play-output { flex: 1; background: var(--code-bg); border: 1px solid var(--border); border-radius: var(--radius); min-height: 400px; }
.play-output pre { margin: 0; padding: 16px; font-family: var(--mono); font-size: 14px; color: #7ee787; white-space: pre-wrap; word-break: break-word; }
.play-output pre.empty { color: var(--text-mute); }
.play-output pre.error { color: var(--red); }
.play-output .run-status { padding: 8px 16px; color: var(--text-mute); font-size: 12.5px; }
.play-templates { display: flex; gap: 8px; flex-wrap: wrap; }
.play-templates button { background: var(--bg-card); border: 1px solid var(--border); color: var(--text-dim); border-radius: 20px; padding: 5px 14px; font-size: 13px; }
.play-templates button:hover { color: var(--text); border-color: var(--primary); }

/* ---------- User box ---------- */
.user-box .btn-user { background: var(--bg-card); border: 1px solid var(--border); color: var(--text); border-radius: 20px; padding: 6px 14px; font-size: 14px; display: flex; align-items: center; gap: 8px; }
.user-box .btn-user .dot { width: 8px; height: 8px; border-radius: 50%; background: var(--green); }
.user-menu { position: relative; display: inline-block; }
.user-drop {
  position: absolute; right: 0; top: calc(100% + 8px); min-width: 190px;
  background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius);
  box-shadow: var(--shadow); padding: 6px; z-index: 60; display: none;
}
.user-menu.open .user-drop { display: block; }
.user-drop a, .user-drop button {
  display: block; width: 100%; text-align: left; background: none; border: none; color: var(--text-dim);
  padding: 8px 12px; border-radius: var(--radius-sm); font-size: 14px;
}
.user-drop a:hover, .user-drop button:hover { background: var(--bg-card-hover); color: var(--text); }

/* ---------- Modal ---------- */
.modal-backdrop { position: fixed; inset: 0; background: rgba(0,0,0,0.6); z-index: 100; display: flex; align-items: center; justify-content: center; padding: 20px; }
.modal { background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius); max-width: 480px; width: 100%; padding: 28px; max-height: 90vh; overflow-y: auto; }

/* ---------- Toast ---------- */
.toast {
  position: fixed; bottom: 24px; left: 50%; transform: translateX(-50%) translateY(80px);
  background: var(--bg-card); border: 1px solid var(--border); border-radius: 20px;
  padding: 12px 22px; box-shadow: var(--shadow); z-index: 200; opacity: 0; transition: all 0.3s;
  font-size: 14.5px; max-width: 90vw;
}
.toast.show { transform: translateX(-50%) translateY(0); opacity: 1; }
.toast.green { border-color: var(--green); color: #b7e7be; }
.toast.red { border-color: var(--red); color: #f5b7b4; }

/* ---------- Misc ---------- */
.pill { display: inline-block; padding: 2px 10px; border-radius: 20px; font-size: 12px; background: var(--primary-bg); color: var(--primary-strong); }
.empty-state { text-align: center; padding: 60px 20px; color: var(--text-mute); }
.loading-box { text-align: center; padding: 60px; color: var(--text-mute); }
.page-title { font-size: 30px; margin: 0 0 6px; }
.page-sub { color: var(--text-dim); margin: 0 0 28px; }
.badge-login-needed { background: var(--orange-bg); color: var(--orange); padding: 12px 16px; border-radius: var(--radius-sm); margin-bottom: 16px; font-size: 14px; }

/* ---------- Footer ---------- */
.site-footer { border-top: 1px solid var(--border); margin-top: 70px; padding: 34px 0; text-align: center; }
.site-footer p { margin: 4px 0; color: var(--text-dim); font-size: 14px; }
.site-footer .footer-sub { color: var(--text-mute); font-size: 13px; }

/* ---------- Responsive ---------- */
@media (max-width: 900px) {
  .learn-layout { grid-template-columns: 1fr; }
  .sidebar { position: static; max-height: none; }
  .play-layout { grid-template-columns: 1fr; }
  .hero h1 { font-size: 32px; }
  .section-title { font-size: 24px; }
  .nav-toggle { display: block; }
  .main-nav {
    position: fixed; top: 62px; left: 0; right: 0; flex-direction: column;
    background: var(--bg); border-bottom: 1px solid var(--border); padding: 10px 16px;
    display: none; gap: 2px;
  }
  .main-nav.open { display: flex; }
  .main-nav a { padding: 12px; font-size: 16px; }
  .header-inner { gap: 12px; }
}