:root {
  --color-bg: #f0f2f5;
  --color-surface: #ffffff;
  --color-border: #d1d5db;
  --color-accent: #4361ee;
  --color-accent-light: #eef0fd;
  --color-text: #1a1d23;
  --color-text-muted: #6b7280;
  --color-error: #dc2626;
  --space-xs: 0.25rem;
  --space-sm: 0.5rem;
  --space-md: 1rem;
  --space-lg: 1.5rem;
  --space-xl: 2rem;
  --space-2xl: 3rem;
  --radius: 8px;
  --radius-sm: 4px;
  --font-mono: 'Courier New', Courier, monospace;
  --shadow: 0 1px 3px rgba(0, 0, 0, 0.08), 0 1px 2px rgba(0, 0, 0, 0.04);
}

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

body {
  font-family: system-ui, -apple-system, 'Segoe UI', sans-serif;
  background: var(--color-bg);
  color: var(--color-text);
  min-height: 100vh;
  padding: var(--space-xl) var(--space-md);
  line-height: 1.5;
}

main {
  max-width: 560px;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  gap: var(--space-xl);
}

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

h1 span {
  color: var(--color-accent);
}

/* --- Cards --- */

.card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  padding: var(--space-lg);
  box-shadow: var(--shadow);
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.card h2 {
  font-size: 1rem;
  font-weight: 600;
  color: var(--color-text);
}

.card-description {
  font-size: 0.875rem;
  color: var(--color-text-muted);
  margin-top: calc(-1 * var(--space-sm));
}

/* --- Input Row --- */

.input-row {
  display: grid;
  grid-template-columns: auto 1fr auto 1fr;
  align-items: center;
  gap: var(--space-sm);
}

.input-row label {
  font-size: 0.8125rem;
  font-weight: 500;
  color: var(--color-text-muted);
  white-space: nowrap;
}

.input-row input[type="number"] {
  width: 100%;
  padding: var(--space-sm) var(--space-md);
  font-size: 1rem;
  font-family: var(--font-mono);
  color: var(--color-text);
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  outline: none;
  transition: border-color 0.15s, box-shadow 0.15s;
  appearance: textfield;
  -moz-appearance: textfield;
}

.input-row input[type="number"]::-webkit-inner-spin-button,
.input-row input[type="number"]::-webkit-outer-spin-button {
  opacity: 1;
}

.input-row input[type="number"]:focus {
  border-color: var(--color-accent);
  box-shadow: 0 0 0 3px color-mix(in srgb, var(--color-accent) 18%, transparent);
}

.input-row input[type="number"]:invalid:not(:placeholder-shown) {
  border-color: var(--color-error);
  box-shadow: 0 0 0 3px color-mix(in srgb, var(--color-error) 15%, transparent);
}

.separator {
  font-size: 1.125rem;
  color: var(--color-text-muted);
  font-weight: 300;
  text-align: center;
}

/* --- Output / Result Display --- */

output {
  display: block;
  background: var(--color-accent-light);
  border-left: 3px solid var(--color-accent);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  padding: var(--space-sm) var(--space-md);
  font-family: var(--font-mono);
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--color-accent);
  letter-spacing: 0.05em;
  min-height: 2.5rem;
}

output.muted {
  font-size: 0.875rem;
  font-family: inherit;
  font-weight: 400;
  color: var(--color-text-muted);
  background: var(--color-bg);
  border-left-color: var(--color-border);
}

/* --- Fieldset within Scaler --- */

fieldset {
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  padding: var(--space-md);
  display: flex;
  flex-direction: column;
  gap: var(--space-sm);
}

legend {
  font-size: 0.8125rem;
  font-weight: 500;
  color: var(--color-text-muted);
  padding: 0 var(--space-xs);
}

/* --- Divider --- */

.divider {
  border: none;
  border-top: 1px solid var(--color-border);
  margin: 0;
}

/* --- Focus-visible for keyboard nav --- */

:focus-visible {
  outline: 2px solid var(--color-accent);
  outline-offset: 2px;
}