:root {
  --bg: #0f172a;         /* slate-900 */
  --panel: #111827;      /* gray-900 */
  --text: #e5e7eb;       /* gray-200 */
  --muted: #9ca3af;      /* gray-400 */
  --primary: #60a5fa;    /* blue-400 */
  --primary-ink: #0b1220;
  --accent: #34d399;     /* emerald-400 */
  --danger: #f87171;     /* red-400 */
  --card: #0b1220;
  --border: #1f2937;     /* gray-800 */
}

[data-theme="light"] {
  --bg: #f8fafc;
  --panel: #ffffff;
  --text: #0f172a;
  --muted: #475569;
  --primary: #2563eb;
  --primary-ink: #ffffff;
  --accent: #059669;
  --danger: #dc2626;
  --card: #ffffff;
  --border: #e5e7eb;
}

* { box-sizing: border-box; }
html, body { height: 100%; }
body {
  margin: 0;
  font-family: system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial, sans-serif;
  background: linear-gradient(180deg, var(--bg), #0b1220 30%);
  color: var(--text);
  line-height: 1.6;
}

.container {
  width: min(960px, 92%);
  margin-inline: auto;
}

.site-header {
  background: radial-gradient(1200px 400px at 50% -100px, #1e293b, transparent 70%);
  border-bottom: 1px solid var(--border);
  padding: 24px 0 8px;
  position: sticky;
  top: 0;
  z-index: 10;
  backdrop-filter: blur(6px);
}
.site-header h1 { margin: 0 0 4px; letter-spacing: 0.3px; }
.tagline { margin: 0 0 12px; color: var(--muted); }

.top-nav { display: flex; gap: 12px; flex-wrap: wrap; margin: 8px 0 12px; }
.top-nav a {
  color: var(--text);
  text-decoration: none;
  padding: 6px 10px;
  border: 1px solid var(--border);
  border-radius: 8px;
}
.top-nav a:hover { background: var(--panel); }

.site-footer { border-top: 1px solid var(--border); padding: 24px 0; margin-top: 40px; color: var(--muted); }

.card {
  background: linear-gradient(180deg, var(--card), var(--panel));
  border: 1px solid var(--border);
  border-radius: 14px;
  padding: 20px 18px;
  margin: 16px 0;
  box-shadow: 0 10px 20px rgba(0,0,0,.15), inset 0 1px 0 rgba(255,255,255,.02);
}

.grid-tiles {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 12px;
}

.tile {
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 12px;
  background: rgba(255,255,255,0.02);
}

.grid-form { display: grid; gap: 12px; align-items: end; }
.grid-form .inline { display: flex; gap: 12px; flex-wrap: wrap; }
.grid-form .field { display: grid; gap: 6px; }
.grid-form .actions { display: flex; gap: 10px; align-items: center; }

label { font-weight: 500; }
input[type="number"] {
  width: 100%;
  padding: 10px 12px;
  border-radius: 10px;
  border: 1px solid var(--border);
  background: var(--panel);
  color: var(--text);
}
input[type="number"]:focus { outline: 2px solid var(--primary); outline-offset: 2px; }

fieldset { border: 1px dashed var(--border); border-radius: 12px; padding: 8px 12px; }
legend { color: var(--muted); padding: 0 6px; }

.btn {
  padding: 8px 14px;
  border-radius: 10px;
  border: 1px solid var(--border);
  background: var(--panel);
  color: var(--text);
  cursor: pointer;
}
.btn:hover { filter: brightness(1.05); }
.btn.primary { background: var(--primary); color: var(--primary-ink); border-color: color-mix(in srgb, var(--primary), black 20%); }
.btn.ghost { background: transparent; }

.result { margin: 4px 0 0; font-weight: 600; }
.ok { color: var(--accent); }
.err { color: var(--danger); }

.muted { color: var(--muted); }

.quiz fieldset { margin: 10px 0; transition: border-color .2s ease; }
.quiz fieldset.is-correct { border-color: color-mix(in srgb, var(--accent), var(--border) 70%); }
.quiz fieldset.is-wrong { border-color: color-mix(in srgb, var(--danger), var(--border) 60%); }
.quiz h3 { margin: 12px 0 6px; color: var(--muted); font-weight: 600; }
.question-figure { border: 1px dashed var(--border); border-radius: 10px; padding: 6px; margin: 6px 0 8px; background: rgba(255,255,255,0.02); }

@media (min-width: 720px) {
  .grid-form {
    grid-template-columns: 1fr 1fr 1fr;
    grid-auto-flow: dense;
  }
  .grid-form fieldset.inline { grid-column: 1 / -1; }
  .grid-form .actions { grid-column: 1 / 2; }
  #ohmResult { grid-column: 1 / -1; }
}