calculator.css

body {
  font-family: "Roboto", sans-serif;
}

.calculator {
  width: 260px;
  margin: 0 auto;
}

button {
  font-family: "Roboto", sans-serif;
  height: 40px;
  padding: 10px;
  border: 0;
  background-color: #1a0dab;
  color: #fff;
  cursor: pointer;
  font-size: 14px;
}

.screen {
  background-color: #beffff;
  height: 50px;
  padding: 10px;
  display: flex;
  justify-content: center;
  align-items: center;
}

.screen span {
  font-size: 20px;
  padding: 0 4px;
}

.controls {
  display: flex;
}

.numbers {
  display: flex;
  flex-wrap: wrap;
  flex: 3;
}

.numbers button {
  flex-basis: 33.33%;
}

.numbers .full-width {
  flex-basis: 100%;
}

.full-width {
  width: 100%;
}

.operators {
  display: flex;
  flex-direction: column;
  flex: 1;
}

.numbers .border-b {
  border-bottom: 1px solid #fff;
}

.numbers .border-sides {
  border-left: 1px solid #fff;
  border-right: 1px solid #fff;
}

.operators button {
  border-left: 1px solid #fff;
  border-bottom: 1px solid #fff;
}