:root {
  --bg: #f4f6fb;
  --surface: #ffffff;
  --surface-2: #f0f3f9;
  --border: #e2e7f0;
  --text: #1c2333;
  --text-muted: #5a6478;
  --accent: #2f6df6;
  --accent-strong: #1e57d6;
  --accent-soft: #e7f0ff;
  --success: #0e9f6e;
  --error: #d64545;
  --shadow: 0 10px 30px rgba(20, 33, 61, 0.08);
  --radius: 14px;
}

:root[data-theme="dark"] {
  --bg: #0f131c;
  --surface: #171d2a;
  --surface-2: #1e2635;
  --border: #2a3242;
  --text: #eaeef7;
  --text-muted: #97a1b5;
  --accent: #5b8cff;
  --accent-strong: #7aa2ff;
  --accent-soft: #1c2a4a;
  --success: #34d399;
  --error: #f87171;
  --shadow: 0 10px 30px rgba(0, 0, 0, 0.4);
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0f131c;
    --surface: #171d2a;
    --surface-2: #1e2635;
    --border: #2a3242;
    --text: #eaeef7;
    --text-muted: #97a1b5;
    --accent: #5b8cff;
    --accent-strong: #7aa2ff;
    --accent-soft: #1c2a4a;
    --success: #34d399;
    --error: #f87171;
    --shadow: 0 10px 30px rgba(0, 0, 0, 0.4);
  }
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
}

.site-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  max-width: 760px;
  margin: 0 auto;
  padding: 18px 20px;
}

.brand { display: flex; align-items: center; gap: 10px; font-weight: 700; }
.logo { font-size: 1.4rem; }
.brand-name { font-size: 1.05rem; letter-spacing: -0.01em; }

.theme-toggle {
  border: 1px solid var(--border);
  background: var(--surface);
  color: var(--text);
  width: 40px;
  height: 40px;
  border-radius: 10px;
  cursor: pointer;
  font-size: 1.1rem;
  display: grid;
  place-items: center;
  transition: background 0.15s, border-color 0.15s;
}
.theme-toggle:hover { border-color: var(--accent); }

main {
  max-width: 760px;
  margin: 0 auto;
  padding: 8px 20px 40px;
}

.intro h1 {
  font-size: 2rem;
  line-height: 1.2;
  margin: 12px 0 8px;
  letter-spacing: -0.02em;
}
.lede { color: var(--text-muted); font-size: 1.02rem; margin: 0 0 24px; }

.calculator {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 22px;
}

.toggles {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
  margin-bottom: 20px;
}

.segmented {
  display: inline-flex;
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 4px;
  gap: 4px;
}
.seg-btn {
  border: none;
  background: transparent;
  color: var(--text-muted);
  padding: 7px 14px;
  border-radius: 7px;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, color 0.15s;
  white-space: nowrap;
}
.seg-btn.active {
  background: var(--surface);
  color: var(--accent);
  box-shadow: 0 1px 3px rgba(0,0,0,0.08);
}

.field-group {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 14px;
  margin-bottom: 14px;
}
.field { display: flex; flex-direction: column; gap: 6px; }
.field-wide { grid-column: 1 / -1; }

label {
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--text-muted);
}
input[type="number"] {
  width: 100%;
  padding: 11px 12px;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: var(--surface-2);
  color: var(--text);
  font-size: 1rem;
  transition: border-color 0.15s, box-shadow 0.15s;
}
input[type="number"]:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-soft);
}

.divider { border: none; border-top: 1px solid var(--border); margin: 18px 0; }
.hidden { display: none !important; }

.results { margin-top: 8px; }
.result-primary {
  background: linear-gradient(135deg, var(--accent-soft), transparent);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 20px;
  text-align: center;
  margin-bottom: 16px;
}
.result-label { display: block; font-size: 0.85rem; color: var(--text-muted); font-weight: 600; }
.result-value {
  display: block;
  font-size: 2.4rem;
  font-weight: 800;
  color: var(--accent);
  letter-spacing: -0.02em;
  margin: 2px 0;
}
.result-note { display: block; font-size: 0.85rem; color: var(--text-muted); }

.result-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 12px;
}
.result-cell {
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 12px 14px;
  display: flex;
  flex-direction: column;
  gap: 3px;
}
.cell-label { font-size: 0.78rem; color: var(--text-muted); font-weight: 600; }
.cell-value { font-size: 1.05rem; font-weight: 700; }

.error {
  margin: 14px 0 0;
  color: var(--error);
  font-size: 0.9rem;
  font-weight: 600;
  text-align: center;
}

.how-it-works, .faq { margin-top: 34px; }
.how-it-works h2, .faq h2 {
  font-size: 1.3rem;
  letter-spacing: -0.01em;
  margin-bottom: 10px;
}
.how-it-works p { color: var(--text-muted); }
code {
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: 5px;
  padding: 1px 6px;
  font-size: 0.9em;
}

.faq details {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 4px 16px;
  margin-bottom: 10px;
}
.faq summary {
  cursor: pointer;
  font-weight: 600;
  padding: 12px 0;
  list-style: none;
}
.faq summary::-webkit-details-marker { display: none; }
.faq summary::after {
  content: "+";
  float: right;
  color: var(--accent);
  font-weight: 700;
}
.faq details[open] summary::after { content: "−"; }
.faq details p { color: var(--text-muted); margin: 0 0 14px; }

.site-footer {
  max-width: 760px;
  margin: 0 auto;
  padding: 24px 20px 40px;
  text-align: center;
  color: var(--text-muted);
  font-size: 0.88rem;
}
.site-footer a { color: var(--accent); text-decoration: none; }
.site-footer a:hover { text-decoration: underline; }

@media (max-width: 520px) {
  .intro h1 { font-size: 1.6rem; }
  .field-group { grid-template-columns: 1fr; }
  .result-grid { grid-template-columns: 1fr; }
  .result-value { font-size: 2rem; }
  .toggles { flex-direction: column; }
  .segmented { width: 100%; }
  .seg-btn { flex: 1; text-align: center; }
}