* {
  box-sizing: border-box;
  font-family: "Poppins", sans-serif;
}

body {
  background-color: #1a1a1a;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
}

.calculator {
  display: flex;
  flex-direction: column;
  background: #222;
  border-radius: 12px;
  overflow: hidden;
  box-shadow: 0 0 20px rgba(0, 0, 0, 0.4);
}

.display {
  background: #333;
  color: #fff;
  padding: 20px;
  text-align: right;
  min-height: 100px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  word-wrap: break-word;
}

.previous-operand {
  color: #aaa;
  font-size: 1.2rem;
}

.current-operand {
  font-size: 2.5rem;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 100px);
  grid-auto-rows: 80px;
}

button {
  border: 1px solid #444;
  background: #2a2a2a;
  color: white;
  font-size: 1.5rem;
  cursor: pointer;
  transition: background 0.2s;
}

button:hover {
  background: #444;
}

.span-two {
  grid-column: span 2;
  background: #ff9500;
  color: #fff;
}

.span-two:hover {
  background: #e08500;
}