/* 
  Design tokens & assumptions
  - Neutral dark text on subtle gray background
  - Single blue accent color for buttons / focus
  - Rounded cards with soft shadow
*/

:root {
  --color-bg: #0f172a;
  --color-surface: #020617;
  --color-surface-alt: #111827;
  --color-border: rgba(148, 163, 184, 0.35);
  --color-primary: #3b82f6;
  --color-primary-soft: rgba(59, 130, 246, 0.15);
  --color-text: #e5e7eb;
  --color-text-muted: #9ca3af;
  --color-error: #f97373;

  --radius-md: 0.75rem;
  --radius-lg: 1.25rem;

  --shadow-soft: 0 18px 40px rgba(15, 23, 42, 0.6);

  --space-xs: 0.375rem;
  --space-sm: 0.75rem;
  --space-md: 1.25rem;
  --space-lg: 2rem;
  --space-xl: 3rem;

  --font-size-sm: 0.875rem;
  --font-size-base: 1rem;
  --font-size-lg: 1.25rem;
  --font-size-xl: 1.75rem;
}

/* === CSS Reset (lightweight) === */

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

html,
body {
  margin: 0;
  padding: 0;
}

body {
  min-height: 100vh;
  font-family: system-ui, -apple-system, BlinkMacSystemFont, "SF Pro Text",
    "Segoe UI", sans-serif;
  font-size: var(--font-size-base);
  color: var(--color-text);
  background:
    radial-gradient(circle at top left, #172554 0, transparent 55%),
    radial-gradient(circle at bottom right, #075985 0, transparent 55%),
    var(--color-bg);
  display: flex;
  align-items: center;
  justify-content: center;
  padding: var(--space-lg) var(--space-md);
}

/* === Main Container === */

.container {
  width: 100%;
  max-width: 32rem;
  margin: 0 auto;
  padding: var(--space-lg) var(--space-md);
  background: linear-gradient(135deg, var(--color-surface), var(--color-surface-alt));
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-soft);
  border: 1px solid rgba(148, 163, 184, 0.25);
}

/* Heading */

.container > h1 {
  margin: 0 0 var(--space-md);
  font-size: var(--font-size-xl);
  font-weight: 650;
  letter-spacing: 0.02em;
  text-align: center;
  color: #f9fafb;
}

/* === Fieldsets (sections) === */

fieldset {
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: var(--space-md);
  margin: 0 0 var(--space-md);
  background: radial-gradient(circle at top left, rgba(148, 163, 184, 0.08), transparent 55%),
              rgba(15, 23, 42, 0.85);
  display: flex;
  flex-direction: column;
  gap: var(--space-sm);
}

/* last fieldset margin */
fieldset:last-of-type {
  margin-bottom: 0;
}

/* Legend */

legend {
  padding: 0 var(--space-xs);
  font-size: var(--font-size-sm);
  text-transform: uppercase;
  letter-spacing: 0.09em;
  color: var(--color-text-muted);
}

/* Labels & Inputs */

label {
  font-size: var(--font-size-sm);
  color: var(--color-text-muted);
}

input[type="number"] {
  width: 100%;
  margin-top: 0.2rem;
  padding: 0.625rem 0.75rem;
  border-radius: 0.55rem;
  border: 1px solid rgba(148, 163, 184, 0.55);
  background-color: rgba(15, 23, 42, 0.9);
  color: var(--color-text);
  font-size: var(--font-size-base);
  outline: none;
  transition:
    border-color 0.15s ease,
    box-shadow 0.15s ease,
    background-color 0.15s ease,
    transform 0.08s ease;
}

input[type="number"]::placeholder {
  color: rgba(148, 163, 184, 0.7);
}

input[type="number"]:hover {
  border-color: var(--color-primary);
  background-color: rgba(15, 23, 42, 0.98);
}

input[type="number"]:focus-visible {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 1px var(--color-primary-soft);
}

/* === Buttons === */

button {
  margin-top: var(--space-sm);
  padding: 0.7rem 1rem;
  border-radius: 999px;
  border: none;
  font-size: var(--font-size-sm);
  font-weight: 600;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  cursor: pointer;
  background: radial-gradient(circle at top left, #60a5fa, #2563eb);
  color: #f9fafb;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 0.4rem;
  width: 100%;
  outline: none;
  transition:
    transform 0.08s ease,
    box-shadow 0.15s ease,
    background-position 0.2s ease,
    filter 0.15s ease;
  background-size: 200% 200%;
}

button:hover {
  transform: translateY(-1px);
  box-shadow: 0 12px 25px rgba(37, 99, 235, 0.5);
  filter: brightness(1.05);
  background-position: 100% 0;
}

button:active {
  transform: translateY(0);
  box-shadow: 0 8px 16px rgba(30, 64, 175, 0.6);
}

button:focus-visible {
  box-shadow:
    0 0 0 1px #0b1120,
    0 0 0 3px var(--color-primary-soft);
}

/* === Outputs === */

output {
  margin-top: var(--space-xs);
  min-height: 1.4em;
  font-size: var(--font-size-lg);
  font-variant-numeric: tabular-nums;
  color: #bfdbfe;
  display: inline-block;
}

/* Muted state if empty (optional visual hint) */
output:empty::before {
  content: "—";
  color: var(--color-text-muted);
}

/* === A11y helpers: error state hook (if JS adds a class) === */

input[type="number"].is-error {
  border-color: var(--color-error);
  box-shadow: 0 0 0 1px rgba(248, 113, 113, 0.4);
}

input[type="number"].is-error::placeholder {
  color: rgba(248, 113, 113, 0.9);
}

/* === Responsive Enhancements === */

/* Medium screens: slightly wider card & more breathing room */
@media (min-width: 640px) {
  .container {
    max-width: 34rem;
    padding: var(--space-xl);
  }

  fieldset {
    padding: var(--space-md) var(--space-lg * 0.7);
  }

  button {
    max-width: 14rem;
  }

  /* align button + output on one row when space allows */
  fieldset {
    gap: var(--space-sm);
  }

  fieldset button + output {
    display: inline-block;
    margin-left: var(--space-sm);
    vertical-align: middle;
  }
}

/* Large screens: larger title and a bit more spacing */
@media (min-width: 1024px) {
  .container {
    max-width: 38rem;
  }

  .container > h1 {
    font-size: 2rem;
    margin-bottom: var(--space-lg);
  }
}