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

html, body { height: 100%; background: #f6f8fa; font-family: 'Segoe UI', system-ui, sans-serif; color: #1f2328; overflow: hidden; }

/* ── Deck & Slides ── */
.deck { position: fixed; inset: 0 0 56px 0; }

.slide {
  display: none; position: absolute; inset: 0;
  padding: 2.5rem 4rem; flex-direction: column;
  justify-content: center; background: #ffffff;
}
.slide.active { display: flex; }

/* Title slide */
.slide.title-slide { align-items: center; text-align: center; background: #f0f6ff; }
.slide.title-slide .lesson-num { font-size: 0.8rem; letter-spacing: 0.15em; text-transform: uppercase; color: #57606a; margin-bottom: 0.75rem; }
.slide.title-slide h1 { font-family: monospace; font-size: 3rem; color: #0969da; letter-spacing: 0.06em; margin-bottom: 0.5rem; }
.slide.title-slide .subtitle { font-size: 1.3rem; color: #57606a; margin-top: 0.5rem; }
.slide.title-slide .big-icon { font-size: 3rem; margin-bottom: 1rem; }

/* Content slides */
.slide h2 { font-size: 1.9rem; color: #0969da; margin-bottom: 1.2rem; padding-bottom: 0.5rem; border-bottom: 1px solid #d0d7de; }

.slide ul { list-style: none; display: flex; flex-direction: column; gap: 0.55rem; }
.slide ul li { font-size: 1.25rem; padding-left: 1.4rem; position: relative; color: #1f2328; line-height: 1.45; }
.slide ul li::before { content: '▸'; position: absolute; left: 0; color: #0969da; }
.slide ul li strong { color: #1f2328; font-weight: 600; }
.slide ul li em { color: #7d4e00; font-style: normal; }

.slide ul.sub { margin-top: 0.3rem; gap: 0.3rem; }
.slide ul.sub li { font-size: 1.05rem; color: #57606a; padding-left: 1.2rem; }
.slide ul.sub li::before { content: '–'; color: #57606a; }

/* Code / cipher blocks */
.example {
  background: #f6f8fa; border: 1px solid #d0d7de;
  border-left: 3px solid #0969da; border-radius: 6px;
  padding: 1rem 1.4rem; margin: 0.8rem 0;
  font-family: 'Consolas', 'Courier New', monospace; font-size: 1.15rem;
  letter-spacing: 0.06em;
}
.example .lbl { font-family: 'Segoe UI', system-ui, sans-serif; font-size: 0.68rem; text-transform: uppercase; letter-spacing: 0.1em; color: #0969da; margin-bottom: 0.35rem; }
.example .ct { color: #cf222e; }
.example .pt { color: #1a7f37; }
.example .key { color: #8250df; }
.example .arr { color: #57606a; margin: 0 0.4rem; }
.example .row { margin: 0.2rem 0; color: #1f2328; }

/* Alphabet mapping table */
.alpha-map { display: grid; grid-template-columns: repeat(13, 1fr); gap: 2px; font-family: monospace; font-size: 0.95rem; margin: 0.8rem 0; }
.alpha-map .cell { text-align: center; padding: 0.25rem 0; }
.alpha-map .cell.top { color: #57606a; font-size: 0.75rem; }
.alpha-map .cell.bot { color: #7d4e00; background: #fff8c5; border-radius: 3px; }

/* Tables */
.slide table { border-collapse: collapse; font-family: monospace; font-size: 1rem; margin: 0.6rem 0; }
.slide table th { background: #f6f8fa; color: #0969da; padding: 0.35rem 0.75rem; border: 1px solid #d0d7de; font-size: 0.75rem; text-transform: uppercase; letter-spacing: 0.05em; }
.slide table td { padding: 0.3rem 0.75rem; border: 1px solid #d0d7de; color: #1f2328; }
.slide table tr:nth-child(even) td { background: #f6f8fa; }

/* Callouts */
.callout { border-radius: 6px; padding: 0.75rem 1.1rem; font-size: 1.1rem; margin: 0.6rem 0; }
.callout.blue   { background: #ddf4ff; border: 1px solid #54aeff; color: #0550ae; }
.callout.green  { background: #dafbe1; border: 1px solid #4ac26b; color: #1a7f37; }
.callout.yellow { background: #fff8c5; border: 1px solid #d4a72c; color: #7d4e00; }
.callout.red    { background: #fff0ee; border: 1px solid #ff8182; color: #cf222e; }

/* Two-column */
.two-col { display: grid; grid-template-columns: 1fr 1fr; gap: 2rem; width: 100%; }

/* Progress bar */
.progress { position: fixed; top: 0; left: 0; height: 3px; background: #0969da; transition: width 0.25s; z-index: 10; }

/* Nav bar */
.slide-nav {
  position: fixed; bottom: 0; left: 0; right: 0; height: 56px;
  background: #ffffff; border-top: 1px solid #d0d7de;
  display: flex; align-items: center; justify-content: center; gap: 1.2rem;
}
.slide-nav a.back { position: absolute; left: 1.5rem; color: #57606a; text-decoration: none; font-size: 0.85rem; }
.slide-nav a.back:hover { color: #0969da; }
.slide-nav button {
  background: #f6f8fa; border: 1px solid #d0d7de; color: #1f2328;
  padding: 0.35rem 1.1rem; border-radius: 6px; font-size: 1.1rem;
  cursor: pointer; transition: background 0.15s;
}
.slide-nav button:hover { background: #e8ecf0; }
.slide-nav button:disabled { opacity: 0.35; cursor: default; }
.slide-nav .counter { font-size: 0.82rem; color: #57606a; min-width: 55px; text-align: center; }