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

:root {
  --bg: #0f1117;
  --surface: #1a1d27;
  --surface2: #22263a;
  --border: #2e3250;
  --accent: #5b6ef5;
  --accent-hover: #7080ff;
  --accent-dim: rgba(91, 110, 245, 0.15);
  --text: #e8eaf0;
  --text-muted: #8b90a8;
  --success: #3ecf8e;
  --danger: #f56565;
  --radius: 10px;
  --radius-sm: 6px;
}

body {
  background: var(--bg);
  color: var(--text);
  font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
  font-size: 15px;
  line-height: 1.5;
  min-height: 100vh;
  padding: 2rem 1rem;
}

.container {
  max-width: 760px;
  margin: 0 auto;
}

header h1 {
  font-size: 1.8rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  margin-bottom: 1.5rem;
  color: var(--text);
}

/* Tabs */
.tabs {
  display: flex;
  gap: 4px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 4px;
  margin-bottom: 1.5rem;
}

.tab-btn {
  flex: 1;
  padding: 0.6rem 1rem;
  border: none;
  border-radius: var(--radius-sm);
  background: transparent;
  color: var(--text-muted);
  font-size: 0.95rem;
  font-weight: 500;
  cursor: pointer;
  transition: background 0.15s, color 0.15s;
}

.tab-btn.active {
  background: var(--accent);
  color: #fff;
}

.tab-btn:not(.active):hover {
  background: var(--surface2);
  color: var(--text);
}

/* Panels */
.tab-panel {
  display: none;
}

.tab-panel.active {
  display: block;
}

.tab-panel h2 {
  font-size: 1.1rem;
  font-weight: 600;
  color: var(--text-muted);
  margin-bottom: 1.25rem;
  letter-spacing: 0.01em;
}

/* Form elements */
.form-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1rem;
  margin-bottom: 1rem;
}

.form-grid.single {
  grid-template-columns: 1fr auto;
  align-items: flex-end;
}

.field-group {
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
}

label {
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

input[type="datetime-local"],
input[type="number"] {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  color: var(--text);
  font-size: 0.95rem;
  padding: 0.6rem 0.8rem;
  width: 100%;
  transition: border-color 0.15s;
  color-scheme: dark;
}

input[type="datetime-local"]:focus,
input[type="number"]:focus {
  outline: none;
  border-color: var(--accent);
}

/* Buttons */
.button-row {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin: 1rem 0;
}

.btn {
  padding: 0.55rem 1.1rem;
  border-radius: var(--radius-sm);
  border: none;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, opacity 0.15s;
  white-space: nowrap;
}

.btn.primary {
  background: var(--accent);
  color: #fff;
}

.btn.primary:hover {
  background: var(--accent-hover);
}

.btn.secondary {
  background: var(--surface2);
  color: var(--text);
  border: 1px solid var(--border);
}

.btn.secondary:hover {
  border-color: var(--accent);
  color: var(--accent);
}

.btn.ghost {
  background: transparent;
  color: var(--text-muted);
  border: 1px solid var(--border);
}

.btn.ghost:hover {
  color: var(--text);
  border-color: var(--text-muted);
}

.btn.inline {
  height: 38px;
  margin-bottom: 0;
}

/* Result card */
.result-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.5rem;
  margin-top: 1rem;
}

.result-card.hidden,
.error.hidden {
  display: none;
}

/* Diff result grid */
.result-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(100px, 1fr));
  gap: 0.75rem;
  margin-bottom: 1.5rem;
}

.result-tile {
  background: var(--surface2);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 1rem 0.75rem;
  text-align: center;
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
}

.tile-value {
  font-size: 1.6rem;
  font-weight: 700;
  color: var(--accent);
  line-height: 1;
  font-variant-numeric: tabular-nums;
}

.tile-label {
  font-size: 0.78rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  font-weight: 600;
}

/* Breakdown table */
.breakdown h3 {
  font-size: 0.85rem;
  font-weight: 700;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin-bottom: 0.75rem;
}

.breakdown-table {
  width: 100%;
  border-collapse: collapse;
}

.breakdown-table td {
  padding: 0.45rem 0;
  font-size: 0.9rem;
}

.breakdown-table td:first-child {
  color: var(--text-muted);
  width: 55%;
}

.breakdown-table td:last-child {
  font-weight: 600;
  font-variant-numeric: tabular-nums;
}

.breakdown-table tr + tr td {
  border-top: 1px solid var(--border);
}

/* Arithmetic units grid */
.units-grid {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  margin-bottom: 1rem;
}

.unit-row {
  display: grid;
  grid-template-columns: 120px 1fr auto;
  align-items: center;
  gap: 1rem;
  padding: 0.65rem 1rem;
  border-bottom: 1px solid var(--border);
}

.unit-row:last-child {
  border-bottom: none;
}

.unit-row.header-row {
  background: var(--surface2);
  font-size: 0.78rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  padding: 0.5rem 1rem;
}

.unit-row label {
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--text);
  text-transform: none;
  letter-spacing: 0;
}

.unit-input {
  max-width: 120px;
}

.op-toggle {
  display: flex;
  gap: 2px;
  background: var(--surface2);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 2px;
}

.op-btn {
  padding: 0.3rem 0.7rem;
  border: none;
  border-radius: 4px;
  background: transparent;
  color: var(--text-muted);
  font-size: 1rem;
  font-weight: 700;
  cursor: pointer;
  transition: background 0.12s, color 0.12s;
  min-width: 32px;
}

.op-btn.active {
  background: var(--accent);
  color: #fff;
}

.op-btn:not(.active):hover {
  background: var(--border);
  color: var(--text);
}

/* Arithmetic result */
.arith-output {
  display: flex;
  flex-direction: column;
  gap: 0;
}

.arith-row {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 0.6rem 0;
  gap: 1rem;
}

.arith-row + .arith-row {
  border-top: 1px solid var(--border);
}

.arith-label {
  font-size: 0.85rem;
  color: var(--text-muted);
  font-weight: 500;
}

.arith-value {
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--text);
  text-align: right;
  font-variant-numeric: tabular-nums;
  word-break: break-all;
}

.arith-row:first-child .arith-value {
  font-size: 1.15rem;
  color: var(--success);
}

/* Error */
.error {
  background: rgba(245, 101, 101, 0.1);
  border: 1px solid rgba(245, 101, 101, 0.35);
  border-radius: var(--radius-sm);
  color: var(--danger);
  font-size: 0.9rem;
  padding: 0.75rem 1rem;
  margin-top: 0.75rem;
}

/* Responsive */
@media (max-width: 520px) {
  .form-grid {
    grid-template-columns: 1fr;
  }

  .form-grid.single {
    grid-template-columns: 1fr;
  }

  .result-grid {
    grid-template-columns: repeat(3, 1fr);
  }

  .unit-row {
    grid-template-columns: 90px 1fr auto;
    gap: 0.5rem;
    padding: 0.65rem 0.75rem;
  }

  header h1 {
    font-size: 1.4rem;
  }
}