body {
  margin: 0;
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  background: linear-gradient(135deg, #0a192f, #1e3a8a);
  font-family: 'Segoe UI', sans-serif;
}

/* Calculator Card */
.calculator {
  background: rgba(255, 255, 255, 0.95);
  padding: 25px;
  border-radius: 20px;
  box-shadow: 0 15px 40px rgba(0, 0, 0, 0.3);
  backdrop-filter: blur(10px);
}

/* Display */
#display {
  width: 100%;
  height: 60px;
  font-size: 26px;
  margin-bottom: 20px;
  text-align: right;
  padding: 10px;
  border: none;
  border-radius: 12px;
  background: #f8fafc;
  color: #0a192f;
  font-weight: bold;
}

/* Buttons Grid */
.buttons {
  display: grid;
  grid-template-columns: repeat(4, 70px);
  gap: 12px;
}

/* Buttons */
button {
  height: 55px;
  font-size: 18px;
  border: none;
  border-radius: 12px;
  cursor: pointer;
  background: #1e3a8a; /* rich blue */
  color: #ffffff;
  transition: all 0.2s ease;
}

/* Hover Effect */
button:hover {
  background: #2563eb;
  transform: translateY(-2px);
}

/* Active Click Effect */
button:active {
  transform: scale(0.95);
}

/* Special Buttons */
button[data-value="C"] {
  background: #ef4444; /* soft red */
}

button[data-value="="] {
  background: #22c55e; /* green highlight */
}

.zero {
  grid-column: span 2;
}