:root {
  --bg: #fafafa;
  --fg: #1a1a1a;
  --accent: #2563eb;
  --accent-light: #dbeafe;
  --border: #e5e7eb;
  --code-bg: #f3f4f6;
  --node-prim: #3b5998;
  --node-const: #7a6c1e;
  --node-input: #2e8b40;
  --node-output: #b33333;
  --wire: #333;
  --success: #16a34a;
  --warning: #ca8a04;
  --info: #0284c7;
}

* { box-sizing: border-box; margin: 0; padding: 0; }

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, sans-serif;
  color: var(--fg);
  background: var(--bg);
  line-height: 1.7;
  max-width: 820px;
  margin: 0 auto;
  padding: 2rem 1.5rem 4rem;
}

h1 { font-size: 2rem; font-weight: 700; margin-bottom: 0.5rem; }
h2 { font-size: 1.5rem; font-weight: 600; margin-top: 2.5rem; margin-bottom: 0.75rem; border-bottom: 2px solid var(--border); padding-bottom: 0.3rem; }
h3 { font-size: 1.15rem; font-weight: 600; margin-top: 1.75rem; margin-bottom: 0.5rem; }

p { margin-bottom: 1rem; }
a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

code {
  font-family: "SF Mono", "Menlo", "Consolas", monospace;
  font-size: 0.9em;
  background: var(--code-bg);
  padding: 0.15em 0.4em;
  border-radius: 4px;
}

pre {
  background: var(--code-bg);
  padding: 1rem 1.25rem;
  border-radius: 8px;
  overflow-x: auto;
  margin-bottom: 1.25rem;
  line-height: 1.5;
  font-size: 0.9rem;
}
pre code { background: none; padding: 0; }

/* Navigation */
nav.top {
  display: flex;
  align-items: center;
  gap: 1.5rem;
  padding-bottom: 1rem;
  border-bottom: 1px solid var(--border);
  margin-bottom: 2rem;
  flex-wrap: wrap;
}
nav.top .logo { font-weight: 700; font-size: 1.2rem; }
nav.top a { font-size: 0.95rem; }

nav.lesson-nav {
  display: flex;
  justify-content: space-between;
  margin-top: 3rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
}
nav.lesson-nav a {
  display: inline-flex;
  align-items: center;
  gap: 0.3rem;
  padding: 0.5rem 1rem;
  border: 1px solid var(--border);
  border-radius: 6px;
  font-size: 0.9rem;
}
nav.lesson-nav a:hover { background: var(--accent-light); text-decoration: none; }

/* Lesson index cards */
.lesson-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 1rem;
  margin-top: 1.5rem;
}
.lesson-card {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1rem 1.25rem;
  transition: border-color 0.15s;
}
.lesson-card:hover { border-color: var(--accent); }
.lesson-card h3 { margin-top: 0; font-size: 1.05rem; }
.lesson-card .num { color: var(--accent); font-weight: 700; margin-right: 0.3rem; }
.lesson-card p { margin-bottom: 0; color: #555; font-size: 0.93rem; }

/* Dataflow diagrams (ASCII art in pre blocks) */
pre.diagram {
  background: #fff;
  border: 1px solid var(--border);
  text-align: center;
  font-size: 0.85rem;
  line-height: 1.6;
}

/* SVG dataflow diagrams */
.diagram-svg {
  display: block;
  max-width: 100%;
  height: auto;
  margin: 1rem auto 1.25rem;
}

/* Concept callouts */
.callout {
  border-left: 4px solid var(--info);
  background: #f0f9ff;
  padding: 0.75rem 1rem;
  margin-bottom: 1.25rem;
  border-radius: 0 6px 6px 0;
}
.callout.tip { border-color: var(--success); background: #f0fdf4; }
.callout.warning { border-color: var(--warning); background: #fefce8; }
.callout strong { display: block; margin-bottom: 0.25rem; }

/* Exercises */
.exercise {
  border: 2px solid var(--accent);
  border-radius: 8px;
  padding: 1rem 1.25rem;
  margin: 1.5rem 0;
}
.exercise h4 {
  color: var(--accent);
  margin-bottom: 0.5rem;
}
.exercise ol, .exercise ul { padding-left: 1.5rem; }
.exercise li { margin-bottom: 0.4rem; }

details {
  margin: 0.75rem 0 1.25rem;
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 0.5rem 1rem;
}
details summary {
  cursor: pointer;
  font-weight: 600;
  color: var(--accent);
  font-size: 0.95rem;
}
details[open] summary { margin-bottom: 0.5rem; }

/* Kbd */
kbd {
  display: inline-block;
  padding: 0.1em 0.5em;
  font-size: 0.85em;
  font-family: inherit;
  background: var(--code-bg);
  border: 1px solid #ccc;
  border-radius: 4px;
  box-shadow: 0 1px 0 #bbb;
}

/* Table */
table {
  width: 100%;
  border-collapse: collapse;
  margin-bottom: 1.25rem;
  font-size: 0.93rem;
}
th, td {
  text-align: left;
  padding: 0.5rem 0.75rem;
  border-bottom: 1px solid var(--border);
}
th { font-weight: 600; background: var(--code-bg); }

/* Hero on index */
.hero {
  text-align: center;
  padding: 2rem 0 1rem;
}
.hero h1 { font-size: 2.5rem; }
.hero .subtitle { font-size: 1.15rem; color: #555; margin-bottom: 1.5rem; }

.badge {
  display: inline-block;
  padding: 0.2em 0.6em;
  font-size: 0.8rem;
  border-radius: 12px;
  font-weight: 600;
}
.badge.beginner { background: #dcfce7; color: #166534; }
.badge.intermediate { background: #fef3c7; color: #92400e; }
.badge.advanced { background: #fee2e2; color: #991b1b; }

footer {
  margin-top: 4rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
  text-align: center;
  font-size: 0.85rem;
  color: #888;
}