style.css

*, *::before, *::after {
    box-sizing: border-box;
    font-family: Arial, sans-serif;
    font-weight: normal;
  }
  
  body {
    padding: 0;
    margin: 0;
    background: linear-gradient(to right, #ff9900, #fffb00);
  }
  
  .calc-container {
    display: grid;
    justify-content: center;
    align-content: center;
    min-height: 100vh;
    grid-template-columns: repeat(4, 90px);
    grid-template-rows: minmax(110px, auto) repeat(6, 80px);
  }
  
  .calc-container > button {
    cursor: pointer;
    font-size: 2rem;
    border: 1px solid white;
    outline: none;
    background-color: rgba(255, 255, 255, .75);
  }
  
  .calc-container > button > span {
    vertical-align: super;
    font-size: 1.2rem;
  }
  
  .calc-container > button:hover {
    background-color: rgba(255, 255, 255, .9);
  }
  
  .span-two {
    grid-column: span 2;
  }
  
  .calc-output {
    display: grid;
    grid-column: 1 / -1;
    background-color: rgba(0, 0, 0, .75);
    display: flex;
    align-items: flex-end;
    justify-content: space-around;
    flex-direction: column;
    padding: 10px;
    word-wrap: break-word;
    word-break: break-all;
  }
  
  .calc-output .previous-operand {
    color: rgba(255, 255, 255, .75);
    font-size: 1.5rem;
  }
  
  .calc-output .current-operand {
    color: white;
    font-size: 2.5rem;
  }