styles.css

html {
  box-sizing: border-box;
}

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

body {
  padding: 0;
  margin: 0;
  font-family: 'Courier New', Courier, monospace;
  font-size: 40px;
}

.wrapper {
  width: 400px;
  background-color: #000000;
  color: #ffffff;
  margin: 50px auto;
  border: 1px solid #000000;
  border-radius: 5px;
}

.screen {
  text-align: right;
  padding: 20px 20px;
}

.calc-button-row {
  display: flex;
  flex-flow: row nowrap;
  justify-content: space-between;
}

.calc-button {
  background-color: #d8d9db;
  color: #000000;
  font-size: 40px;
  height: 100px;
  border: solid 1px #000000;
  border-radius: 3px;
  flex-basis: 25%;
}

.calc-button:last-child {
  background-color: #df974c;
  color: #ffffff;
}

.calc-button:last-child:hover {
  background-color: #dfb07e;
}

.calc-button:last-child:active {
  background-color: #dd8d37;
}

.calc-button:hover {
  background-color: #ebebeb;
}

.calc-button:active {
  background-color: ##bbbcbe;
}

.double {
  flex-basis: 50%;
}

.triple {
  flex-basis: 75%;
}