:root {
  --bg: #0f172a;
  --panel: #111827;
  --display: #1f2937;
  --button: #e5e7eb;
  --button-operator: #f59e0b;
  --button-function: #94a3b8;
  --button-equals: #22c55e;
  --text: #f8fafc;
  --shadow: rgba(15, 23, 42, 0.25);
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  font-family: Arial, Helvetica, sans-serif;
}

body {
  min-height: 100vh;
  display: grid;
  place-items: center;
  background: linear-gradient(135deg, #020817, #1e293b);
  color: var(--text);
}

.calculator {
  width: min(92vw, 360px);
  background: rgba(17, 24, 39, 0.9);
  border-radius: 22px;
  box-shadow: 0 20px 40px var(--shadow);
  padding: 18px;
  border: 1px solid rgba(148, 163, 184, 0.2);
}

.display {
  min-height: 96px;
  background: var(--display);
  border-radius: 14px;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  align-items: flex-end;
  padding: 16px 16px;
  border: 1px solid rgba(148, 163, 184, 0.2);
  gap: 6px;
}

.expression,
.result {
  width: 100%;
  text-align: right;
  overflow-wrap: anywhere;
  word-break: break-all;
}

.expression {
  font-size: 1rem;
  opacity: 0.75;
  min-height: 1.2em;
}

.result {
  font-size: clamp(2rem, 4vw, 2.6rem);
  min-height: 1.5em;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 12px;
  margin-top: 18px;
}

.btn {
  border: none;
  border-radius: 14px;
  padding: 20px 12px;
  font-size: 1.3rem;
  font-weight: 700;
  cursor: pointer;
  transition: transform 0.12s ease, filter 0.12s ease;
  color: #0f172a;
}

.btn:hover {
  filter: brightness(1.08);
}

.btn:active {
  transform: translateY(1px) scale(0.99);
}

.number {
  background: var(--button);
}

.operator {
  background: var(--button-operator);
  color: white;
}

.function {
  background: var(--button-function);
  color: white;
}

.equals {
  background: var(--button-equals);
  color: white;
  grid-column: span 2;
}

.zero {
  grid-column: span 2;
}