* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: 'Segoe UI', sans-serif;
  background: radial-gradient(circle at top, #1f2933, #0b0f14);
  min-height: 100vh;
  align-items: center;
  justify-content: center;
}

.app {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
}

.calculator {
  width: 340px;
  background: #0f172a;
  padding: 20px;
  border-radius: 20px;
  box-shadow: 0 25px 60px rgba(0, 0, 0, 0.6);
}

#screen {
  width: 100%;
  height: 60px;
  margin-bottom: 15px;
  background: #020617;
  color: #22c55e;
  border: none;
  border-radius: 12px;
  font-size: 26px;
  padding: 10px 14px;
  text-align: right;
  letter-spacing: 1px;
}

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

button {
  height: 55px;
  border-radius: 14px;
  border: none;
  font-size: 16px;
  background: #1e293b;
  color: #e5e7eb;
  cursor: pointer;
  transition: all 0.15s ease;
  box-shadow: inset 0 -3px 0 rgba(0,0,0,0.4);
}

button:hover {
  transform: translateY(-2px);
  background: #334155;
}

button:active {
  transform: translateY(0);
  box-shadow: inset 0 2px 4px rgba(0,0,0,0.6);
}

/* Operators */
.operator {
  background: linear-gradient(135deg, #fb923c, #f97316);
  color: #020617;
}

.operator:hover {
  background: linear-gradient(135deg, #fdba74, #fb923c);
}

/* Functions */
.function {
  background: #475569;
}

.equal {
  background: linear-gradient(135deg, #22c55e, #16a34a);
  color: #022c22;
  font-size: 20px;
}

/* Wide zero */
.zero {
  grid-column: span 2;
}