:root {
  --bg: #fbfbf9;
  --surface: #ffffff;
  --text: #1d1d1f;
  --muted: #6e6e73;
  --accent: #0066cc;
  --border: #e5e5ea;
  --code-bg: #f5f5f7;
  --success: #34c759;
  --warning: #ff9500;
  --font-mono: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  line-height: 1.6;
  color: var(--text);
  background: var(--bg);
  margin: 0;
  padding: 2rem 1rem;
}

.container {
  max-width: 820px;
  margin: 0 auto;
  background: var(--surface);
  padding: 3rem 2.5rem;
  border-radius: 12px;
  box-shadow: 0 4px 20px rgba(0, 0, 0, 0.05);
  border: 1px solid var(--border);
}

h1 {
  font-size: 2rem;
  font-weight: 700;
  margin-top: 0;
  border-bottom: 2px solid var(--border);
  padding-bottom: 0.8rem;
  color: #111;
}

h2 {
  font-size: 1.4rem;
  margin-top: 2rem;
  color: #222;
}

h3 {
  font-size: 1.15rem;
  margin-top: 1.5rem;
  color: #333;
}

p, li {
  font-size: 1rem;
}

pre {
  background: var(--code-bg);
  padding: 1rem 1.2rem;
  border-radius: 8px;
  overflow-x: auto;
  font-family: var(--font-mono);
  font-size: 0.9rem;
  border: 1px solid var(--border);
}

code {
  font-family: var(--font-mono);
  background: var(--code-bg);
  padding: 0.2rem 0.4rem;
  border-radius: 4px;
  font-size: 0.9em;
}

.box {
  padding: 1.2rem;
  margin: 1.5rem 0;
  border-radius: 8px;
  border-left: 4px solid var(--accent);
  background: #f0f7ff;
}

.box-exercise {
  border-left-color: var(--success);
  background: #f2faf4;
}

.box-tip {
  border-left-color: var(--warning);
  background: #fffaf0;
}

.box h4 {
  margin-top: 0;
  margin-bottom: 0.5rem;
  font-size: 1.05rem;
}

.quiz {
  margin-top: 2rem;
  padding: 1.5rem;
  background: #fafafa;
  border: 1px solid var(--border);
  border-radius: 8px;
}

.quiz button {
  display: block;
  width: 100%;
  text-align: left;
  margin: 0.5rem 0;
  padding: 0.8rem 1rem;
  border: 1px solid var(--border);
  border-radius: 6px;
  background: white;
  cursor: pointer;
  font-size: 0.95rem;
  transition: all 0.2s ease;
}

.quiz button:hover {
  background: #f0f0f5;
}

.quiz .feedback {
  margin-top: 1rem;
  font-weight: 600;
  display: none;
}

.footer {
  margin-top: 3rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
  font-size: 0.9rem;
  color: var(--muted);
  display: flex;
  justify-content: space-between;
}