/* Space-themed dark mode */

:root {
  --bg-deep: #05071a;
  --bg-mid: #0b1030;
  --bg-card: #111738;
  --bg-card-hover: #161e48;
  --border: #2a346b;
  --border-glow: #4f6ad6;
  --fg: #e7ecff;
  --fg-dim: #a5b0d8;
  --muted: #7a85b3;
  --accent: #67e8f9;        /* cyan */
  --accent-2: #a78bfa;      /* violet */
  --accent-hover: #a5f3fc;
  --good: #6ee7b7;
  --bad: #fca5a5;
  --shadow: 0 4px 16px rgba(0, 0, 0, 0.45), 0 0 0 1px rgba(79, 106, 214, 0.08);
  --glow: 0 0 24px rgba(103, 232, 249, 0.25);
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  min-height: 100vh;
  color: var(--fg);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  font-size: 16px;
  line-height: 1.55;
  background-color: var(--bg-deep);
  background-image:
    /* nebula glows */
    radial-gradient(ellipse 60% 40% at 15% 10%, rgba(122, 75, 199, 0.18), transparent 60%),
    radial-gradient(ellipse 50% 35% at 85% 20%, rgba(56, 189, 248, 0.12), transparent 60%),
    radial-gradient(ellipse 70% 50% at 50% 100%, rgba(167, 139, 250, 0.10), transparent 60%),
    /* stars (data URL of randomly placed dots) */
    url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='600' height='600'><g fill='white'><circle cx='40' cy='80' r='0.7' opacity='0.8'/><circle cx='130' cy='40' r='0.5' opacity='0.6'/><circle cx='220' cy='120' r='0.9' opacity='0.9'/><circle cx='310' cy='60' r='0.4' opacity='0.5'/><circle cx='400' cy='160' r='0.7' opacity='0.7'/><circle cx='490' cy='90' r='0.5' opacity='0.6'/><circle cx='570' cy='200' r='0.6' opacity='0.7'/><circle cx='60' cy='240' r='0.5' opacity='0.5'/><circle cx='180' cy='310' r='0.8' opacity='0.8'/><circle cx='270' cy='270' r='0.4' opacity='0.5'/><circle cx='360' cy='340' r='0.7' opacity='0.7'/><circle cx='450' cy='280' r='0.5' opacity='0.6'/><circle cx='530' cy='380' r='0.6' opacity='0.7'/><circle cx='90' cy='400' r='0.5' opacity='0.5'/><circle cx='160' cy='480' r='0.8' opacity='0.85'/><circle cx='250' cy='440' r='0.5' opacity='0.6'/><circle cx='340' cy='520' r='0.6' opacity='0.7'/><circle cx='430' cy='460' r='0.4' opacity='0.55'/><circle cx='510' cy='540' r='0.7' opacity='0.75'/><circle cx='580' cy='480' r='0.5' opacity='0.6'/><circle cx='20' cy='350' r='0.4' opacity='0.5'/><circle cx='110' cy='180' r='0.5' opacity='0.55'/><circle cx='380' cy='420' r='0.5' opacity='0.6'/></g></svg>");
  background-repeat: no-repeat, no-repeat, no-repeat, repeat;
}

a { color: var(--accent); text-decoration: none; }
a:hover { color: var(--accent-hover); text-decoration: underline; }

.topbar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 14px 24px;
  background: rgba(11, 16, 48, 0.92);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 10;
}
.topbar h1 { font-size: 18px; margin: 0; font-weight: 600; letter-spacing: 0.02em; }
.topbar h1 a { color: var(--fg); background: linear-gradient(90deg, var(--accent), var(--accent-2)); -webkit-background-clip: text; background-clip: text; -webkit-text-fill-color: transparent; }
.topbar nav a { margin-left: 18px; font-size: 14px; color: var(--fg-dim); }
.topbar nav a:hover { color: var(--accent); }

.container {
  max-width: 880px;
  margin: 0 auto;
  padding: 28px 24px 48px;
}

h2 {
  margin-top: 0;
  font-weight: 600;
  letter-spacing: -0.01em;
  font-size: 26px;
}
h3 { font-weight: 600; letter-spacing: -0.005em; }

.card-list { list-style: none; padding: 0; }
.card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 18px 22px;
  box-shadow: var(--shadow);
  margin-bottom: 14px;
  transition: border-color 0.15s, box-shadow 0.15s, transform 0.15s;
}
.card:hover {
  border-color: var(--border-glow);
  box-shadow: var(--shadow), var(--glow);
  transform: translateY(-1px);
}
.card a { color: var(--fg); display: block; }
.card a:hover { text-decoration: none; }
.card h3 { margin: 0 0 4px 0; font-size: 18px; }
.card p { margin: 0; color: var(--fg-dim); font-size: 14px; }

.filters {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 16px 20px;
  margin-bottom: 16px;
  box-shadow: var(--shadow);
}
.filters details { margin-bottom: 10px; }
.filters summary {
  cursor: pointer;
  font-weight: 600;
  padding: 6px 0;
  user-select: none;
  color: var(--fg);
}
.filters summary::marker, .filters summary::-webkit-details-marker { color: var(--accent); }
.filter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(180px, 1fr));
  gap: 6px 16px;
  padding: 10px 0 14px 4px;
}
.filter-grid label {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 14px;
  cursor: pointer;
  color: var(--fg-dim);
}
.filter-grid label:hover { color: var(--fg); }
.filter-grid input[type="checkbox"] {
  accent-color: var(--accent);
}
.filter-actions {
  display: flex;
  align-items: center;
  gap: 12px;
  padding-top: 12px;
  border-top: 1px solid var(--border);
}

button, .button {
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  color: #0b1030;
  border: none;
  padding: 10px 18px;
  border-radius: 8px;
  cursor: pointer;
  font-size: 14px;
  font-weight: 600;
  letter-spacing: 0.01em;
  transition: transform 0.1s, box-shadow 0.15s, filter 0.15s;
  display: inline-block;
}
button:hover, .button:hover {
  filter: brightness(1.1);
  box-shadow: var(--glow);
  text-decoration: none;
}
button:active, .button:active { transform: translateY(1px); }

button.secondary,
#reveal-btn,
#clear-filters {
  background: transparent;
  color: var(--accent);
  border: 1px solid var(--border);
  font-weight: 500;
}
#reveal-btn:hover, #clear-filters:hover {
  background: rgba(103, 232, 249, 0.08);
  border-color: var(--border-glow);
  color: var(--accent-hover);
  box-shadow: none;
}

/* Start practice CTA on browse page */
.session-cta {
  text-align: center;
  padding: 28px 20px;
  background: linear-gradient(135deg, rgba(103, 232, 249, 0.06), rgba(167, 139, 250, 0.06));
  border: 1px solid var(--border);
  border-radius: 12px;
  margin-top: 20px;
}
.session-cta .count {
  font-size: 32px;
  font-weight: 700;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
  margin-bottom: 2px;
}
.session-cta .count-label {
  font-size: 14px;
  color: var(--muted);
  margin-bottom: 18px;
}
.session-cta .start-btn {
  font-size: 16px;
  padding: 12px 28px;
}
.session-cta.empty {
  background: transparent;
  border: 1px dashed var(--border);
}
.session-cta.empty .count {
  background: none;
  color: var(--muted);
  -webkit-text-fill-color: var(--muted);
  font-size: 20px;
  font-weight: 500;
}

.session-cta .upcoming {
  margin-top: 22px;
  text-align: left;
  font-size: 13px;
  color: var(--fg-dim);
}
.session-cta .upcoming-title {
  text-transform: uppercase;
  letter-spacing: 0.08em;
  font-size: 11px;
  color: var(--muted);
  margin-bottom: 8px;
}
.session-cta .upcoming ol {
  list-style: none;
  padding: 0;
  margin: 0;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 6px 16px;
}
.session-cta .upcoming li {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 4px 0;
}

.badge {
  display: inline-block;
  font-size: 11px;
  padding: 3px 9px;
  border-radius: 999px;
  background: rgba(122, 133, 179, 0.15);
  color: var(--fg-dim);
  border: 1px solid rgba(122, 133, 179, 0.25);
  letter-spacing: 0.02em;
  font-weight: 500;
}
.badge.diff-1 { background: rgba(110, 231, 183, 0.12); color: #6ee7b7; border-color: rgba(110, 231, 183, 0.3); }
.badge.diff-2 { background: rgba(125, 211, 252, 0.12); color: #7dd3fc; border-color: rgba(125, 211, 252, 0.3); }
.badge.diff-3 { background: rgba(253, 224, 71, 0.12); color: #fde047; border-color: rgba(253, 224, 71, 0.3); }
.badge.diff-4 { background: rgba(251, 146, 60, 0.12); color: #fb923c; border-color: rgba(251, 146, 60, 0.3); }
.badge.diff-5 { background: rgba(252, 165, 165, 0.12); color: var(--bad); border-color: rgba(252, 165, 165, 0.3); }
.badge.format-mc { background: rgba(167, 139, 250, 0.12); color: var(--accent-2); border-color: rgba(167, 139, 250, 0.3); }
.badge.format-fr { background: rgba(103, 232, 249, 0.12); color: var(--accent); border-color: rgba(103, 232, 249, 0.3); }

/* Problem view */
.problem-view .problem-meta {
  color: var(--fg-dim);
  font-size: 13px;
  margin-bottom: 10px;
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  align-items: center;
}
.problem-view .problem-topic {
  font-size: 22px;
  font-weight: 600;
  color: var(--fg);
  margin: 4px 0 16px;
  letter-spacing: -0.01em;
}
.problem-prompt {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 22px 24px;
  margin-bottom: 18px;
  box-shadow: var(--shadow);
  font-size: 17px;
  line-height: 1.6;
  color: var(--fg);
}
.problem-prompt .katex { color: var(--fg); }
.answer-form { margin-bottom: 24px; }
.answer-form input[type="text"] {
  font-size: 16px;
  padding: 10px 14px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--bg-mid);
  color: var(--fg);
  width: 100%;
  max-width: 360px;
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
}
.answer-form input[type="text"]:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px rgba(103, 232, 249, 0.15);
}
.answer-form label { color: var(--fg-dim); display: block; margin-bottom: 4px; }

.mc-choices { list-style: none; padding: 0; }
.mc-choices li { margin: 8px 0; }
.mc-choices label {
  display: flex;
  align-items: flex-start;
  gap: 12px;
  padding: 12px 16px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--bg-card);
  cursor: pointer;
  color: var(--fg);
  transition: border-color 0.1s, background 0.1s;
}
.mc-choices label:hover {
  background: var(--bg-card-hover);
  border-color: var(--border-glow);
}
.mc-choices input { margin-top: 5px; accent-color: var(--accent); }
.mc-choices input:checked + span { color: var(--accent-hover); }

.answer-actions { margin-top: 14px; display: flex; gap: 10px; }
.feedback { margin-top: 14px; font-weight: 600; }
.feedback.correct { color: var(--good); }
.feedback.wrong { color: var(--bad); }

.solution {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 18px 22px;
  margin-bottom: 16px;
  box-shadow: var(--shadow);
}
.solution h3 { margin-top: 0; color: var(--accent); }
.solution .katex { color: var(--fg); }
.self-grade { margin-top: 14px; padding-top: 14px; border-top: 1px solid var(--border); }
.self-grade p { color: var(--fg-dim); margin: 0 0 8px 0; font-size: 14px; }
.self-grade button { margin-right: 8px; }
.self-grade button[data-grade="wrong"] {
  background: transparent;
  color: var(--bad);
  border: 1px solid rgba(252, 165, 165, 0.4);
}
.self-grade button[data-grade="wrong"]:hover {
  background: rgba(252, 165, 165, 0.08);
  box-shadow: none;
}

.problem-nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-top: 18px;
}
.problem-nav a { color: var(--fg-dim); font-size: 14px; }
.problem-nav a:hover { color: var(--accent); }
.problem-nav .queue-pos { color: var(--muted); font-size: 12px; }

.hidden { display: none !important; }

.empty {
  text-align: center;
  color: var(--muted);
  padding: 32px;
}

/* Override KaTeX colors for legibility on dark bg */
.katex { color: var(--fg) !important; }
.katex .frac-line { border-color: var(--fg) !important; }

/* Auth UI in topbar */
.auth-slot {
  display: inline-flex;
  align-items: center;
  gap: 12px;
  margin-left: 18px;
  font-size: 14px;
}
.auth-user {
  color: var(--fg);
  max-width: 180px;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}
.auth-link {
  color: var(--fg-dim);
}
.auth-link.sign-in {
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  color: #0b1030 !important;
  padding: 4px 12px;
  border-radius: 6px;
  font-weight: 600;
}
.auth-link.sign-in:hover { filter: brightness(1.1); text-decoration: none; box-shadow: var(--glow); }

/* Per-unit progress count in the unit filter */
.unit-progress {
  color: var(--muted);
  font-size: 12px;
  margin-left: 4px;
}

/* Previously-solved checkmark in the upcoming list */
.session-cta .upcoming li .check {
  color: var(--good);
  font-weight: 700;
  margin-left: 6px;
}