* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font: normal 25px Arial, sans-serif;
    -webkit-touch-callout: none;
    -webkit-user-select: none;
    user-select: none;
  }
  
  html {
    height: 100%;
    background: linear-gradient(to right, #26d0ce, #1a2980);
  }
  
  h1 {
    margin: 0.5em 0;
    font-size: calc(3vw + 2rem);
    font-weight: 100;
    text-align: center;
    text-transform: uppercase;
    letter-spacing: -0.05em;
    color: white;
  }
  
  #calculator {
    width: 325px;
    height: auto;
    overflow: hidden;
    margin: 100px auto;
    padding: 20px 20px 9px;
    position: relative;
    background: rgba(0, 0, 0, 0.3);
    border-radius: 3px;
    box-shadow: 0px 10px 15px rgba(0, 0, 0, 0.2);
  }
  
  .top span.clear {
    position: absolute;
    z-index: 6;
  }
  
  .top .screen {
    height: 80px;
    width: 100%;
    position: absolute;
    top: 0;
    left: 0;
    padding: 20px 10px;
    background: rgba(0, 0, 0, 0.2);
    font-size: 27px;
    font-weight: normal;
    line-height: 40px;
    color: white;
    text-shadow: 1px 1px 2px rgba(0, 0, 0, 0.2);
    text-align: right;
    letter-spacing: 1px;
  }
  
  .keys,
  .top {
    overflow: hidden;
  }
  
  .keys span,
  .top span.clear {
    float: left;
    position: relative;
    top: 0;
    cursor: pointer;
    width: 66px;
    height: 56px;
    background: rgba(0, 0, 0, 0.5);
    border-radius: 3px;
    box-shadow: 0px 4px rgba(0, 0, 0, 0.2);
    margin: 0 7px 11px 0;
    color: #ccc;
    line-height: 56px;
    text-align: center;
    user-select: none;
    transition: all 0.2s ease;
  }
  
  .keys span.operator {
    background: rgba(250, 100, 0, 0.3);
    margin-right: 0;
  }
  
  .keys span.eval {
    color: #888e5f;
  }
  
  .top span.clear {
    background: transparent;
    box-shadow: none;
    color: #aaa;
    width: 56px;
    left: 5px;
  }
  
  .keys span:hover {
    background: rgba(255, 255, 255, 0.8);
    color: #333;
  }
  
  .keys span.eval:hover {
    background: #abb850;
    color: #ffffff;
  }
  
  .top span.clear:hover {
    border-radius: 190px;
    background: rgba(0, 0, 0, 0.1);
    color: white;
  }
  
  .keys span:active {
    box-shadow: 0px 0px rgba(0, 0, 0, 0.2);
    top: 4px;
  }
  
  .keys span.eval:active {
    box-shadow: 0px 0px #717a33;
    top: 4px;
  }
  
  .top span.clear:active {
    top: 4px;
    box-shadow: 0px 0px #d3545d;
  }