style.css

*, *::before, *::after {
  box-sizing: border-box;
  font-family: Gotham Rounded, sans-serif;
  font-weight: normal;
}

body {
  padding: 0;
  margin: 0;
  background: linear-gradient(to right, #00AAFF, #00ff6c);
}

.calculator {
  height: 15rem;
  position: relative;
}

.calculator-keys {
  position:absolute;
  top: 5rem;
  left: 4.5rem;
  display: grid;
  justify-content: center;
  align-content: center;
  grid-template-columns: repeat(4, 4rem);
  grid-template-rows: minmax(4rem, auto) repeat(4, 4rem);
}

.calculator-keys > button {
  cursor: pointer;
  font-size: 1rem;
  border: 1px solid white;
  outline: none;
  background-color: #9ad18b;;
}

.calculator-keys > button:hover {
  background-color: rgba(255, 255, 255, .90);
}

.calculator-keys > button.is-depressed {
background-color: rgba(255, 255, 255, .90);
}

.span-two {
  grid-column: span 2;
}

.output {
  top: 1.5rem;
  left: 4.5rem;
  min-height: 2rem;
  width: 16rem;
  background-color: rgba(0, 0, 0, .75);
  display: flex;
  position: absolute;
  flex-direction: column;
  align-items: flex-end;
  justify-content: space-around;
  padding: 10px;
  word-wrap: break-word;
  word-break: break-all;
}

.output .current-operand {
  color: white;
  font-size: 2.5rem;
}