body {
  background: #f4f6fb;
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
}

.calculator-container {
  background: #fff;
  border-radius: 18px;
  box-shadow: 0 8px 32px 0 rgba(31, 38, 135, 0.2);
  padding: 2.5rem 2rem 2rem 2rem;
  max-width: 350px;
  width: 100%;
}

#display {
  font-size: 2rem;
  padding: 16px;
  border-radius: 8px;
  border: 1px solid #e0e0e0;
  margin-bottom: 1.5rem;
  background: #f8fafc;
  text-align: right;
  box-shadow: 0 2px 8px rgba(0,0,0,0.03);
}

.btn {
  font-size: 1.2rem;
  border-radius: 8px;
  margin-bottom: 0.5rem;
  box-shadow: 0 1px 2px rgba(0,0,0,0.04);
  transition: background 0.2s, color 0.2s;
}
.btn:active {
  background: #e2e6ea;
}
.btn-equals {
  background: #4f8cff;
  color: #fff;
  font-weight: bold;
}
.btn-equals:hover {
  background: #2563eb;
}
.btn-op {
  background: #f1f5f9;
  color: #2563eb;
  font-weight: bold;
}
.btn-op:hover {
  background: #e0e7ef;
}
.btn-clear {
  background: #f87171;
  color: #fff;
}
.btn-clear:hover {
  background: #dc2626;
}