app.css

body {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

.calculator {
  display: flex;
  flex-direction: column;
}

#display {
  display: flex;
  justify-content: flex-end;
  align-items: flex-end;
  padding: 0 15px 5px;
  height: calc(30vh - 5px);
  background-color: #222;
  font-family: Helvetica, sans-serif;
  font-size: 4em;
  font-weight: 100;
  color: white;
}

#inputs {
  display: flex;
  flex-direction: column;
  height: 70vh;
}

.row {
  display: flex;
  height: 20%;
}

input {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 25%;
  outline: 1px solid #333;
  border: none;
  font-size: 2em;
  font-weight: lighter;
  background-color: #EEE;
  cursor: pointer;
}

input:active {
  font-size: 1.8em;
  background-color: #AAA;
}

#zero {
  width: 50%;
}