:root {
  --bg: #0f1117;
  --surface: #1a1d27;
  --surface-2: #232734;
  --border: #2e3444;
  --text: #e4e7ef;
  --text-dim: #a0a7ba;
  --accent: #6ea8fe;
  --accent-2: #7ee0b8;
  --warn: #f0b656;
  --danger: #f08585;
  --code-bg: #12141c;
  --easy: #4caf7d;
  --med: #d9a441;
  --hard: #d96565;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  line-height: 1.65;
  font-size: 17px;
}

.container {
  max-width: 860px;
  margin: 0 auto;
  padding: 48px 28px 96px;
}

/* Header */
.day-header {
  border-bottom: 2px solid var(--border);
  padding-bottom: 20px;
  margin-bottom: 8px;
}
.day-badge {
  display: inline-block;
  background: var(--accent);
  color: #0f1117;
  font-weight: 700;
  font-size: 13px;
  letter-spacing: 0.5px;
  padding: 4px 12px;
  border-radius: 999px;
  text-transform: uppercase;
}
.day-header h1 {
  font-size: 34px;
  margin: 14px 0 6px;
  line-height: 1.2;
}
.day-meta {
  color: var(--text-dim);
  font-size: 15px;
}

/* Section cards */
.section {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 24px 28px;
  margin: 24px 0;
}
.section h2 {
  margin-top: 0;
  font-size: 22px;
  display: flex;
  align-items: center;
  gap: 10px;
}
.section h2 .icon { font-size: 20px; }
.section h3 {
  font-size: 18px;
  color: var(--accent);
  margin: 22px 0 8px;
}

/* Mental model highlight */
.mental-model {
  border-left: 4px solid var(--accent-2);
  background: var(--surface-2);
  padding: 16px 20px;
  border-radius: 0 8px 8px 0;
  margin: 16px 0;
}
.mental-model strong { color: var(--accent-2); }

/* Complexity table */
table {
  width: 100%;
  border-collapse: collapse;
  margin: 14px 0;
  font-size: 15px;
}
th, td {
  text-align: left;
  padding: 10px 14px;
  border-bottom: 1px solid var(--border);
}
th {
  color: var(--text-dim);
  font-weight: 600;
  text-transform: uppercase;
  font-size: 12px;
  letter-spacing: 0.5px;
}
code.inline {
  background: var(--code-bg);
  padding: 2px 7px;
  border-radius: 5px;
  font-family: "SF Mono", "Fira Code", Consolas, monospace;
  font-size: 14px;
  color: var(--accent-2);
}

/* Code blocks */
pre {
  background: var(--code-bg);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 20px;
  overflow-x: auto;
  font-family: "SF Mono", "Fira Code", Consolas, monospace;
  font-size: 14px;
  line-height: 1.55;
}
pre code { color: #d6deeb; }
.tok-key { color: #c792ea; }
.tok-type { color: #82aaff; }
.tok-str { color: #c3e88d; }
.tok-com { color: #6a7489; font-style: italic; }
.tok-num { color: #f78c6c; }

/* Pitfalls */
.pitfall {
  border-left: 4px solid var(--danger);
  background: rgba(240, 133, 133, 0.08);
  padding: 12px 18px;
  border-radius: 0 8px 8px 0;
  margin: 12px 0;
}
.pitfall strong { color: var(--danger); }
.tip {
  border-left: 4px solid var(--warn);
  background: rgba(240, 182, 86, 0.08);
  padding: 12px 18px;
  border-radius: 0 8px 8px 0;
  margin: 12px 0;
}
.tip strong { color: var(--warn); }

/* Problems */
.problem {
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 14px 18px;
  background: var(--surface-2);
  border-radius: 8px;
  margin: 10px 0;
}
.problem .lc-num {
  font-weight: 700;
  color: var(--accent);
  min-width: 62px;
}
.problem .lc-title { flex: 1; }
.problem .hint {
  display: block;
  color: var(--text-dim);
  font-size: 13px;
  margin-top: 2px;
}
.tag {
  font-size: 12px;
  font-weight: 700;
  padding: 3px 10px;
  border-radius: 999px;
  text-transform: uppercase;
}
.tag.easy { background: rgba(76,175,125,0.18); color: var(--easy); }
.tag.med  { background: rgba(217,164,65,0.18); color: var(--med); }
.tag.hard { background: rgba(217,101,101,0.18); color: var(--hard); }

/* Nav */
.nav {
  display: flex;
  justify-content: space-between;
  margin-top: 40px;
  padding-top: 20px;
  border-top: 1px solid var(--border);
}
.nav a {
  color: var(--accent);
  text-decoration: none;
  font-weight: 600;
}
.nav a:hover { text-decoration: underline; }
.nav a.disabled { color: var(--text-dim); pointer-events: none; }

ul { padding-left: 22px; }
li { margin: 6px 0; }
a { color: var(--accent); }

/* Proof / theorem blocks */
.proof {
  border-left: 4px solid var(--accent);
  background: rgba(110, 168, 254, 0.07);
  padding: 14px 20px;
  border-radius: 0 8px 8px 0;
  margin: 16px 0;
}
.proof .label {
  display: block;
  font-weight: 700;
  color: var(--accent);
  text-transform: uppercase;
  font-size: 12px;
  letter-spacing: 0.5px;
  margin-bottom: 6px;
}
.proof .qed { float: right; color: var(--text-dim); }

.theorem {
  border-left: 4px solid var(--accent-2);
  background: var(--surface-2);
  padding: 14px 20px;
  border-radius: 0 8px 8px 0;
  margin: 16px 0;
  font-style: italic;
}
.theorem .label {
  display: block;
  font-weight: 700;
  color: var(--accent-2);
  font-style: normal;
  text-transform: uppercase;
  font-size: 12px;
  letter-spacing: 0.5px;
  margin-bottom: 6px;
}

/* References */
.refs { font-size: 14px; }
.refs li { margin: 8px 0; color: var(--text-dim); }
.refs .cite { color: var(--text); font-weight: 600; }

/* Invariant callout */
.invariant {
  border: 1px dashed var(--accent-2);
  border-radius: 8px;
  padding: 12px 18px;
  margin: 14px 0;
  background: rgba(126, 224, 184, 0.05);
}
.invariant strong { color: var(--accent-2); }

/* Math-ish inline */
.math { font-family: "SF Mono", "Fira Code", Consolas, monospace; font-size: 15px; color: var(--accent-2); }

/* Persistent home button (fixed, on every day page) */
.home-btn {
  position: fixed;
  top: 20px;
  left: 20px;
  z-index: 100;
  display: inline-flex;
  align-items: center;
  gap: 7px;
  background: var(--surface-2);
  border: 1px solid var(--border);
  color: var(--text);
  text-decoration: none;
  font-size: 14px;
  font-weight: 600;
  padding: 8px 14px;
  border-radius: 999px;
  transition: border-color .15s, background .15s;
}
.home-btn:hover { border-color: var(--accent); background: var(--surface); }
.home-btn .home-icon { font-size: 15px; }

@media (max-width: 720px) {
  .home-btn { top: 12px; left: 12px; padding: 6px 12px; font-size: 13px; }
}

/* Problem variance section */
.variance {
  border-left: 4px solid var(--warn);
  background: rgba(240, 182, 86, 0.06);
  padding: 4px 20px 12px;
  border-radius: 0 8px 8px 0;
  margin: 16px 0;
}
.variance h3 { color: var(--warn); }
.variance .disguise {
  font-weight: 600;
  color: var(--text);
}
.variance table td:first-child { color: var(--warn); font-weight: 600; }