:root {
  --primary: #4f46e5;
  --primary-dark: #3730a3;
  --accent: #f59e0b;
  --good: #16a34a;
  --good-bg: #bbf7d0;
  --bad: #dc2626;
  --bad-bg: #fecaca;
  --bg: #f5f7ff;
  --card: #ffffff;
  --text: #1f2937;
  --muted: #6b7280;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: "Comic Sans MS", "Trebuchet MS", system-ui, -apple-system, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
}

/* ---------- Header ---------- */
.hero {
  background: linear-gradient(135deg, #4f46e5 0%, #7c3aed 50%, #ec4899 100%);
  color: white;
  padding: 2rem 1rem 1rem;
  text-align: center;
}
.hero h1 {
  margin: 0;
  font-size: 2.6rem;
  letter-spacing: 1px;
  text-shadow: 2px 2px 0 rgba(0,0,0,0.15);
}
.subtitle {
  margin: 0.25rem 0 1rem;
  font-size: 1.1rem;
  opacity: 0.95;
}
.topnav {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.5rem;
  margin-top: 0.5rem;
}
.topnav a {
  background: rgba(255,255,255,0.18);
  color: white;
  padding: 0.5rem 0.9rem;
  border-radius: 999px;
  text-decoration: none;
  font-weight: bold;
  font-size: 0.95rem;
  transition: background 0.2s;
}
.topnav a:hover { background: rgba(255,255,255,0.35); }

/* ---------- Sections ---------- */
main { max-width: 980px; margin: 0 auto; padding: 1rem; }

.lesson-block {
  background: var(--card);
  border-radius: 18px;
  padding: 1.5rem;
  margin: 1.5rem 0;
  box-shadow: 0 4px 14px rgba(0,0,0,0.08);
}
.lesson-block.alt { background: #fff7ed; }
.lesson-block.test { background: #ecfeff; }

.lesson-block h2 {
  margin-top: 0;
  font-size: 1.8rem;
  color: var(--primary-dark);
  display: flex;
  align-items: center;
  gap: 0.6rem;
}
.step {
  display: inline-flex;
  width: 2.2rem;
  height: 2.2rem;
  background: var(--accent);
  color: white;
  border-radius: 50%;
  align-items: center;
  justify-content: center;
  font-size: 1.2rem;
  font-weight: bold;
}

.lead { font-size: 1.1rem; }

/* ---------- Video ---------- */
.video-wrap {
  position: relative;
  padding-bottom: 56.25%;
  height: 0;
  border-radius: 14px;
  overflow: hidden;
  margin: 1rem 0;
  box-shadow: 0 4px 14px rgba(0,0,0,0.15);
}
.video-wrap iframe {
  position: absolute;
  top: 0; left: 0;
  width: 100%; height: 100%;
  border: 0;
}

/* ---------- Info box ---------- */
.info-box {
  background: #eef2ff;
  border-left: 6px solid var(--primary);
  border-radius: 10px;
  padding: 1rem 1.25rem;
  margin: 1rem 0;
}
.info-box h3 { margin-top: 0; }
.example { background: white; padding: 0.6rem 0.8rem; border-radius: 8px; }

/* ---------- Binary demo ---------- */
.binary-demo {
  background: #fffbeb;
  border: 3px dashed var(--accent);
  border-radius: 14px;
  padding: 1rem;
  text-align: center;
  margin: 1rem 0;
}
.bit-row {
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
  gap: 0.4rem;
  margin: 0.5rem 0 1rem;
}
.bit-cell {
  width: 70px;
  background: white;
  border-radius: 10px;
  padding: 0.4rem;
  box-shadow: 0 3px 6px rgba(0,0,0,0.1);
  cursor: pointer;
  user-select: none;
  transition: transform 0.1s;
}
.bit-cell:hover { transform: translateY(-2px); }
.bit-cell .place {
  font-size: 0.85rem;
  color: var(--muted);
  font-weight: bold;
}
.bit-cell .bit {
  font-size: 2rem;
  font-weight: bold;
  color: var(--text);
  background: #f3f4f6;
  border-radius: 6px;
  margin-top: 0.3rem;
}
.bit-cell.on .bit {
  background: var(--good);
  color: white;
}
.result {
  font-size: 1.3rem;
  font-weight: bold;
  margin: 0.6rem 0;
}
.result span {
  display: inline-block;
  background: white;
  padding: 0.2rem 0.7rem;
  border-radius: 8px;
  margin-left: 0.4rem;
  color: var(--primary-dark);
  font-family: "Courier New", monospace;
}
.working {
  background: white;
  padding: 0.5rem;
  border-radius: 8px;
  font-family: "Courier New", monospace;
  margin-bottom: 0.5rem;
  color: var(--primary-dark);
  font-weight: bold;
}

/* ---------- Challenge widget ---------- */
.challenge-block {
  background: #ecfeff;
  border: 3px solid #06b6d4;
  border-radius: 14px;
  padding: 1rem 1.25rem;
  margin: 1.25rem 0;
  text-align: center;
}
.challenge-block h3 { margin-top: 0; color: #0e7490; }
.challenge-info {
  display: flex;
  justify-content: space-between;
  background: white;
  padding: 0.5rem 0.9rem;
  border-radius: 8px;
  font-weight: bold;
  margin: 0.5rem 0;
}
.challenge-prompt {
  font-size: 1.4rem;
  margin: 0.6rem 0;
}
.challenge-prompt strong, .challenge-prompt code {
  background: #fef3c7;
  padding: 0.15rem 0.6rem;
  border-radius: 8px;
  color: #92400e;
  font-size: 1.5rem;
}
.ch-input {
  font-size: 1.5rem;
  font-family: "Courier New", monospace;
  padding: 0.6rem 0.8rem;
  border: 2px solid #d1d5db;
  border-radius: 8px;
  outline: none;
  text-align: center;
  width: 200px;
  margin: 0.4rem 0;
}
.ch-input:focus { border-color: var(--primary); }
.ch-input.correct { background: var(--good-bg); border-color: var(--good); }
.ch-input.wrong   { background: var(--bad-bg);  border-color: var(--bad); }
.feedback {
  min-height: 1.5rem;
  font-weight: bold;
  margin: 0.4rem 0;
}
.feedback.good { color: var(--good); }
.feedback.bad  { color: var(--bad); }
.challenge-block .btn-primary[disabled] {
  background: #c7d2fe;
  cursor: not-allowed;
}
.challenge-block.done {
  background: linear-gradient(135deg, #bbf7d0, #fef3c7);
  border-color: var(--good);
}

/* ---------- Buttons ---------- */
.btn-primary, .btn-secondary, .btn-play {
  display: inline-block;
  border: none;
  border-radius: 999px;
  padding: 0.8rem 1.6rem;
  font-size: 1.1rem;
  font-weight: bold;
  cursor: pointer;
  text-decoration: none;
  transition: transform 0.1s, box-shadow 0.2s;
  margin: 0.4rem 0.2rem;
  font-family: inherit;
}
.btn-primary {
  background: var(--primary);
  color: white;
}
.btn-primary:hover { background: var(--primary-dark); }
.btn-secondary {
  background: #e5e7eb;
  color: var(--text);
}
.btn-secondary:hover { background: #d1d5db; }
.btn-play {
  background: linear-gradient(135deg, #f59e0b, #ef4444);
  color: white;
  font-size: 1.3rem;
  padding: 1rem 2rem;
  box-shadow: 0 6px 14px rgba(239,68,68,0.4);
}
.btn-play:hover { transform: translateY(-2px); box-shadow: 0 8px 18px rgba(239,68,68,0.5); }

/* ---------- How-to + prize ---------- */
.how-to ol { line-height: 1.9; }
.prize-box {
  background: linear-gradient(135deg, #fde68a, #fbbf24);
  border-radius: 14px;
  padding: 1rem;
  display: flex;
  align-items: center;
  gap: 1rem;
  margin: 1rem 0;
  box-shadow: 0 4px 8px rgba(0,0,0,0.1);
}
.prize-box .trophy { font-size: 2.5rem; }
.prize-box p { margin: 0; }

/* ---------- Quiz / test ---------- */
.q {
  background: white;
  padding: 0.9rem 1rem;
  margin: 0.6rem 0;
  border-radius: 10px;
  box-shadow: 0 2px 5px rgba(0,0,0,0.05);
}
.q label {
  display: block;
  font-weight: bold;
  margin-bottom: 0.4rem;
}
.q.mcq label {
  display: block;
  font-weight: normal;
  background: #f9fafb;
  padding: 0.5rem 0.7rem;
  margin: 0.3rem 0;
  border-radius: 6px;
  cursor: pointer;
}
.q.mcq label:hover { background: #f3f4f6; }
.q.mcq.correct { background: var(--good-bg); border: 2px solid var(--good); }
.q.mcq.wrong { background: var(--bad-bg); border: 2px solid var(--bad); }
.q code {
  background: #f3f4f6;
  padding: 0.15rem 0.4rem;
  border-radius: 4px;
  font-size: 1rem;
}
.quiz-input {
  width: 100%;
  font-size: 1.3rem;
  font-family: "Courier New", monospace;
  padding: 0.6rem 0.8rem;
  border: 2px solid #d1d5db;
  border-radius: 8px;
  outline: none;
  transition: background 0.2s, border-color 0.2s;
}
.quiz-input:focus { border-color: var(--primary); }
.quiz-input.correct {
  background: var(--good-bg);
  border-color: var(--good);
}
.quiz-input.wrong {
  background: var(--bad-bg);
  border-color: var(--bad);
}
.hint { color: var(--muted); font-style: italic; margin: 0.3rem 0; }

/* ---------- Score ---------- */
.score-box {
  margin-top: 1.5rem;
  background: linear-gradient(135deg, #4f46e5, #7c3aed);
  color: white;
  border-radius: 14px;
  padding: 1.5rem;
  text-align: center;
  box-shadow: 0 6px 16px rgba(79,70,229,0.3);
}
.score-box.hidden { display: none; }
.score-num {
  font-size: 3rem;
  font-weight: bold;
  margin: 0.5rem 0;
}
.score-num span { color: var(--accent); }

/* ---------- Footer ---------- */
footer {
  text-align: center;
  padding: 1.5rem;
  color: var(--muted);
  font-size: 0.9rem;
}

/* ---------- Mobile ---------- */
@media (max-width: 600px) {
  .hero h1 { font-size: 2rem; }
  .bit-cell { width: 56px; }
  .bit-cell .bit { font-size: 1.5rem; }
  .lesson-block h2 { font-size: 1.4rem; }
}