html {
  font-size: 10px;
  box-sizing: border-box;
}

*, *::before, *::after {
  margin: 0;
  padding: 0;
  box-sizing: inherit;
}

.calculator {
  border: 1px solid #1E1E1E;
  border-radius: 5px;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 300px;
}

.calculator-screen {
  width: 100%;
  font-size: 5rem;
  height: 80px;
  border: none;
  background-color: #5D5A58;
  color: #fff;
  text-align: right;
  padding-right: 20px;
  padding-left: 30px;
}

button {
  height: 60px;
  border: 1px solid #5E5C5C;
  font-size: 2rem;
  color: #fff;
  background-color: #848281;
}

button:focus {
  outline: none;
}

.operator, .equal-sign {
  background-color: #FF9D00;
}

.clear, .plus-minus, .percent {
  background-color: #6E6D6D;
}

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

.btn-lg {
  grid-column: 1 / 3;
}