* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  background-color: #0f172a;
}

.calculator {
  background-color: #1e293b;
  padding: 24px;
  border-radius: 16px;
  box-shadow: 0 10px 25px rgba(0, 0, 0, 0.4);
  width: 100%;
  max-width: 340px;
}

.calculator-screen {
  width: 100%;
  height: 60px;
  border: none;
  background-color: #0f172a;
  color: #ffffff;
  text-align: right;
  padding: 10px 15px;
  font-size: 2rem;
  border-radius: 8px;
  margin-bottom: 20px;
}

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

button {
  height: 55px;
  border: none;
  background-color: #334155;
  color: #ffffff;
  font-size: 1.25rem;
  border-radius: 8px;
  cursor: pointer;
  transition: background-color 0.2s ease;
}

button:active {
  transform: scale(0.96);
}

button.operator {
  background-color: #3b82f6;
  color: #ffffff;
}

button.equal-sign {
  background-color: #10b981;
  color: #ffffff;
  grid-column: span 2;
}