body {
  margin: 0;
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  background: linear-gradient(135deg, #0f172a, #1e293b);
  font-family: 'Poppins', sans-serif;
}

.calculator {
  background: rgba(255, 255, 255, 0.05);
  border-radius: 20px;
  padding: 25px;
  box-shadow: 0 10px 30px rgba(0, 0, 0, 0.6);
  width: 340px;
  backdrop-filter: blur(12px);
  border: 1px solid rgba(255, 255, 255, 0.1);
}

#display {
  width: 90%;
  height: 75px;
  font-size: 32px;
  font-weight: 600;
  border: none;
  border-radius: 15px;
  margin-bottom: 20px;
  padding: 15px;
  text-align: right;
  background: linear-gradient(145deg, #0f172a, #1e293b);
  color: #e2e8f0;
  box-shadow: inset 2px 2px 6px rgba(0,0,0,0.6),
              inset -2px -2px 6px rgba(255,255,255,0.05);
  outline: none;
  letter-spacing: 1px;
}

#display:focus {
  border: 2px solid #6366f1;
  box-shadow: 0 0 12px #6366f1;
}

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

.btn {
  height: 65px;
  font-size: 20px;
  border: none;
  border-radius: 12px;
  background: rgba(255, 255, 255, 0.08);
  color: #fff;
  cursor: pointer;
  transition: 0.2s;
  box-shadow: 0 4px 8px rgba(0,0,0,0.4);
}

.btn:hover {
  background: rgba(255, 255, 255, 0.2);
  transform: translateY(-2px);
}

.op {
  background: #6366f1;
}

.op:hover {
  background: #4f46e5;
}

.eq {
  background: #22c55e;
  grid-column: span 1;
}

.eq:hover {
  background: #16a34a;
}

.zero {
  grid-column: span 2;
}