:root {
  color-scheme: dark;
  --bg: hsl(30 30% 10%);
  --bg2: hsl(25 55% 18%);

  --panel: hsl(0 0% 8% / 0.85);
  --panelBorder: hsl(0 0% 100% / 0.08);

  --displayBg: hsl(0 0% 10% / 0.9);
  --text: hsl(0 0% 98%);
  --muted: hsl(0 0% 80% / 0.7);

  --key: hsl(0 0% 22%);
  --keyHover: hsl(0 0% 28%);
  --keyDown: hsl(0 0% 32%);
  --op: hsl(31 100% 52% / 0.92);
  --opHover: hsl(31 100% 58% / 0.95);
  --action: hsl(0 0% 35%);
  --actionHover: hsl(0 0% 42%);
  --equal: hsl(150 65% 36% / 0.95);
  --equalHover: hsl(150 65% 40% / 0.98);

  --radius: 18px;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  display: grid;
  place-items: center;
  padding: 24px;
  font-family:
    system-ui,
    -apple-system,
    Segoe UI,
    Roboto,
    Arial,
    sans-serif;
  background:
    radial-gradient(900px 700px at 20% 10%, var(--bg2), transparent 60%),
    radial-gradient(900px 700px at 80% 90%, hsl(35 65% 20%), transparent 55%),
    var(--bg);
  color: var(--text);
}

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

.app {
  width: min(440px, 100%);
}

.calculator {
  background: var(--panel);
  border: 1px solid var(--panelBorder);
  border-radius: var(--radius);
  overflow: hidden;
  box-shadow:
    0 18px 40px hsl(0 0% 0% / 0.45),
    inset 0 1px 0 hsl(0 0% 100% / 0.06);
  backdrop-filter: blur(10px);
}

.display {
  padding: 18px 18px 14px;
  background: linear-gradient(to bottom, var(--displayBg), hsl(0 0% 6% / 0.9));
  border-bottom: 1px solid var(--panelBorder);
}

.expression {
  min-height: 1.2em;
  font-size: 0.95rem;
  color: var(--muted);
  text-align: right;
  letter-spacing: 0.02em;
  user-select: none;
}

.value {
  width: 100%;
  margin-top: 8px;
  padding: 0;
  font-size: clamp(2.4rem, 7vw, 3.4rem);
  line-height: 1.15;
  text-align: right;
  border: none;
  outline: none;
  background: transparent;
  color: var(--text);
  font-variant-numeric: tabular-nums;
}

.keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
  padding: 16px;
}

.key {
  border: none;
  border-radius: 999px;
  height: clamp(56px, 12vw, 76px);
  padding: 0;
  cursor: pointer;
  color: var(--text);
  background: var(--key);
  font-size: clamp(1.25rem, 4.6vw, 1.8rem);
  font-weight: 700;
  letter-spacing: 0.02em;
  box-shadow:
    inset 0 1px 0 hsl(0 0% 100% / 0.08),
    0 10px 18px hsl(0 0% 0% / 0.25);
  transition:
    transform 90ms ease,
    background-color 120ms ease,
    box-shadow 120ms ease;
  user-select: none;
  -webkit-tap-highlight-color: transparent;
}

.key:hover {
  background: var(--keyHover);
}

.key:active {
  background: var(--keyDown);
  transform: translateY(1px) scale(0.99);
  box-shadow:
    inset 0 1px 0 hsl(0 0% 100% / 0.06),
    0 6px 12px hsl(0 0% 0% / 0.22);
}

.key:focus-visible {
  outline: 3px solid hsl(31 100% 60% / 0.65);
  outline-offset: 2px;
}

.key--op {
  background: var(--op);
}

.key--op:hover {
  background: var(--opHover);
}

.key--action {
  background: var(--action);
}

.key--action:hover {
  background: var(--actionHover);
}

.key--equal {
  background: var(--equal);
}

.key--equal:hover {
  background: var(--equalHover);
}

.key--zero {
  grid-column: span 2;
  text-align: left;
  padding-left: 28px;
}

@media (max-width: 360px) {
  .keys {
    gap: 8px;
    padding: 14px;
  }

  .key--zero {
    padding-left: 22px;
  }
}