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

/* ── Theme variables ─────────────────────────────────────────────────────── */

:root {
  --color-bg:             #f0f2f5;
  --color-surface:        #ffffff;
  --color-surface-raised: #f8f9fb;
  --color-border:         #dde1e9;
  --color-text:           #1a1d23;
  --color-text-muted:     #6b7280;
  --color-primary:        #2563eb;
  --color-primary-hover:  #1d4ed8;
  --color-secondary-bg:   #f3f4f6;
  --color-secondary-hover:#e5e7eb;
  --color-error:          #dc2626;
  --color-error-bg:       #fef2f2;
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.08), 0 1px 2px rgba(0,0,0,0.05);
  --radius:    8px;
  --radius-sm: 5px;
}

[data-theme="dark"] {
  --color-bg:             #0d0f16;
  --color-surface:        #161822;
  --color-surface-raised: #1e2030;
  --color-border:         #2d3148;
  --color-text:           #e2e4f0;
  --color-text-muted:     #8b90a8;
  --color-primary:        #4f8ef7;
  --color-primary-hover:  #6ba3ff;
  --color-secondary-bg:   #1e2030;
  --color-secondary-hover:#262840;
  --color-error:          #f87171;
  --color-error-bg:       #2d1a1a;
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.35), 0 1px 2px rgba(0,0,0,0.25);
}

/* ── Base ────────────────────────────────────────────────────────────────── */

html {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  font-size: 14px;
  line-height: 1.5;
}

body {
  background: var(--color-bg);
  color: var(--color-text);
  min-height: 100vh;
  transition: background 0.2s, color 0.2s;
}

/* ── Header ──────────────────────────────────────────────────────────────── */

.site-header {
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  box-shadow: var(--shadow-sm);
  position: sticky;
  top: 0;
  z-index: 10;
}

.header-content {
  max-width: 1440px;
  margin: 0 auto;
  padding: 0.875rem 1.5rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

h1 {
  font-size: 1.2rem;
  font-weight: 600;
  letter-spacing: -0.01em;
}

.theme-btn {
  background: var(--color-secondary-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  color: var(--color-text);
  cursor: pointer;
  font-size: 0.8125rem;
  padding: 0.375rem 0.8rem;
  transition: background 0.15s, border-color 0.15s;
  white-space: nowrap;
}

.theme-btn:hover {
  background: var(--color-secondary-hover);
}

/* ── Main two-column layout ──────────────────────────────────────────────── */

.layout {
  max-width: 1440px;
  margin: 0 auto;
  padding: 1.5rem;
  display: grid;
  grid-template-columns: 340px 1fr;
  gap: 1.5rem;
  align-items: start;
}

/* ── Input panel ─────────────────────────────────────────────────────────── */

.panel-inputs {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  box-shadow: var(--shadow-sm);
  padding: 1.25rem;
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.form-group {
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
}

.form-group > label {
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--color-text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

select,
input[type="number"] {
  background: var(--color-surface-raised);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  color: var(--color-text);
  font-size: 0.9375rem;
  padding: 0.5rem 0.625rem;
  width: 100%;
  transition: border-color 0.15s, box-shadow 0.15s;
}

select {
  appearance: none;
  -webkit-appearance: none;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='8' viewBox='0 0 12 8'%3E%3Cpath fill='%238b90a8' d='M6 8 0 0h12z'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 0.7rem center;
  padding-right: 2.25rem;
  cursor: pointer;
}

select:focus,
input[type="number"]:focus {
  outline: none;
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px color-mix(in srgb, var(--color-primary) 18%, transparent);
}

/* Fieldset */
fieldset {
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  padding: 0.875rem;
}

fieldset legend {
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--color-text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  padding: 0 0.375rem;
}

/* Radio toggle */
.radio-group {
  display: flex;
  gap: 1.25rem;
  margin-bottom: 0.75rem;
}

.radio-label {
  display: flex;
  align-items: center;
  gap: 0.375rem;
  font-size: 0.875rem;
  cursor: pointer;
}

.radio-label input[type="radio"] {
  accent-color: var(--color-primary);
  cursor: pointer;
}

/* Subgroup (inside fieldset or #spectrum-params) */
.form-subgroup {
  display: flex;
  flex-direction: column;
  gap: 0.275rem;
  margin-top: 0.5rem;
}

.form-subgroup:first-child {
  margin-top: 0;
}

.form-subgroup > label {
  font-size: 0.8125rem;
  color: var(--color-text-muted);
}

/* Min / max row */
.minmax-row {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.75rem;
  align-items: start;
}

.minmax-row .form-subgroup {
  margin-top: 0;
}

/* Spectrum param inputs (generated dynamically) */
#spectrum-params .form-group > label {
  font-size: 0.8125rem;
  font-weight: 500;
  color: var(--color-text-muted);
  text-transform: none;
  letter-spacing: 0;
}

/* Placeholder notice (JONSWAP) */
.param-notice {
  background: var(--color-surface-raised);
  border: 1px dashed var(--color-border);
  border-radius: var(--radius-sm);
  color: var(--color-text-muted);
  font-size: 0.875rem;
  line-height: 1.55;
  padding: 0.75rem 1rem;
  text-align: center;
}

/* Buttons */
.action-buttons {
  display: flex;
  flex-direction: column;
  gap: 0.625rem;
}

.btn-primary {
  background: var(--color-primary);
  border: none;
  border-radius: var(--radius-sm);
  color: #fff;
  cursor: pointer;
  font-size: 0.9375rem;
  font-weight: 500;
  padding: 0.625rem 1rem;
  transition: background 0.15s;
  width: 100%;
}

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

.btn-secondary {
  background: var(--color-secondary-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  color: var(--color-text);
  cursor: pointer;
  font-size: 0.9375rem;
  font-weight: 500;
  padding: 0.625rem 1rem;
  transition: background 0.15s;
  width: 100%;
}

.btn-secondary:hover:not(:disabled) {
  background: var(--color-secondary-hover);
}

.btn-secondary:disabled {
  cursor: not-allowed;
  opacity: 0.4;
}

/* Error */
.error-msg {
  background: var(--color-error-bg);
  border: 1px solid var(--color-error);
  border-radius: var(--radius-sm);
  color: var(--color-error);
  font-size: 0.875rem;
  line-height: 1.5;
  padding: 0.6rem 0.75rem;
}

/* Frequency count display */
.freq-count-display {
  color: var(--color-text-muted);
  font-size: 0.8rem;
  margin-top: 0.2rem;
  min-height: 1.1em;
}

/* ── Chart panel ─────────────────────────────────────────────────────────── */

/* Chart controls (x-axis switcher) */
.chart-controls {
  display: flex;
  align-items: center;
  gap: 1rem;
  padding-bottom: 0.875rem;
  border-bottom: 1px solid var(--color-border);
  margin-bottom: 1rem;
  width: 100%;
  flex-wrap: wrap;
}

.chart-controls-label {
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--color-text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  white-space: nowrap;
}

.panel-chart {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  box-shadow: var(--shadow-sm);
  padding: 1.25rem;
  min-height: 440px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
}

.chart-container {
  width: 100%;
  flex: 1;
  min-height: 380px;
  position: relative;
}

.chart-placeholder {
  color: var(--color-text-muted);
  font-size: 0.9375rem;
  text-align: center;
  padding: 1rem;
}

.chart-placeholder strong {
  color: var(--color-text);
}

/* ── References footer ───────────────────────────────────────────────────── */

.references {
  background: var(--color-surface);
  border-top: 1px solid var(--color-border);
  margin-top: 0.5rem;
  padding: 2rem 1.5rem 2.5rem;
}

.references-inner {
  max-width: 1440px;
  margin: 0 auto;
}

.references h2 {
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--color-text-muted);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin-bottom: 1.25rem;
}

.references-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(360px, 1fr));
  gap: 1rem;
}

.reference-card {
  background: var(--color-surface-raised);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  padding: 1.125rem 1.25rem;
}

.reference-card h3 {
  font-size: 0.9375rem;
  font-weight: 600;
  margin-bottom: 0.875rem;
}

.reference-card .formula {
  margin-bottom: 0.875rem;
  overflow-x: auto;
  padding: 0.25rem 0;
}

.reference-card .citation {
  color: var(--color-text-muted);
  font-size: 0.8125rem;
  font-style: italic;
  line-height: 1.65;
}

/* KaTeX colour inherit */
.katex { color: var(--color-text); }

/* ── Period indicator cards ──────────────────────────────────────────────── */

.period-cards {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  gap: 0.625rem;
  margin-top: 1rem;
  width: 100%;
}

.period-card {
  background: var(--color-surface-raised);
  border: 1px solid var(--color-border);
  border-left: 3px solid transparent;
  border-radius: var(--radius-sm);
  padding: 0.6rem 0.75rem;
  display: flex;
  flex-direction: column;
  gap: 0.15rem;
}

.period-card-symbol {
  font-size: 1rem;
  font-weight: 700;
  line-height: 1.2;
}

.period-card-name {
  font-size: 0.7rem;
  color: var(--color-text-muted);
  line-height: 1.3;
}

.period-card-value {
  font-size: 0.9375rem;
  font-weight: 600;
  margin-top: 0.2rem;
}

.period-card-sub {
  font-size: 0.75rem;
  color: var(--color-text-muted);
}

/* ── Responsive ──────────────────────────────────────────────────────────── */

@media (max-width: 900px) {
  .layout {
    grid-template-columns: 1fr;
  }

  .chart-container {
    min-height: 320px;
  }

  .references-grid {
    grid-template-columns: 1fr;
  }
}