* {
  margin: 0;
  padding: 0;
  font-family: "Segoe UI", Calibri, sans-serif;
  box-sizing: border-box;
}

body {
  background: linear-gradient(135deg, #232526, #414345);
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}

.main {
  text-align: center;
}

.main h1 {
  margin-bottom: 20px;
  color: #a4e640;
  text-shadow: 1px 1px 4px rgba(0, 0, 0, 0.6);
}

.calculator {
  background: #2c3e50;
  padding: 20px;
  border-radius: 15px;
  box-shadow: 0px 8px 20px rgba(0, 0, 0, 0.6);
  display: inline-block;
}

#display {
  width: 100%;
  height: 50px;
  margin-bottom: 15px;
  text-align: right;
  font-size: 22px;
  padding: 10px;
  border: none;
  border-radius: 8px;
  outline: none;
  background: #ecf0f1;
  box-shadow: inset 2px 2px 6px rgba(0, 0, 0, 0.2);
}

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

button {
  height: 60px;
  font-size: 20px;
  font-weight: 600;
  border: none;
  border-radius: 12px;
  background: #34495e;
  color: white;
  box-shadow: 2px 2px 8px rgba(0, 0, 0, 0.5);
  cursor: pointer;
  transition: all 0.2s ease-in-out;
}

button:hover {
  transform: scale(1.05);
  background: #3d566e;
}

.btn-green {
  background: #a4e640;
  color: #2c3e50;
  font-weight: bold;
}

.btn-green:hover {
  background: #8ecf36;
}