* {
  box-sizing: border-box;
  font-family: 'Open Sans', Helvetica, Arial, sans-serif;
  font-size: 24px;
  font-size: 4vw;
  font-weight: 300;
  margin: 0;
  padding: 0;
}
html {
  height: 100%;
  background: radial-gradient(circle, #fff, #ddd);
  background-size: cover;
}
#calculator {
  width: 12em;
  height: auto;
  background: linear-gradient(#4389A2, #5C258D);
  margin: 1em auto;
}
.top span.clear {
  float: left;
  box-shadow: 0 4px #ff7c87;
  color: #FFF;
}
.top .input {
  height: 3em;
  width: 9em;
  padding-right: 1em;
  float: right;
  color: #FFF;
  line-height: 3em;
  text-align: right;
  letter-spacing: 1px;
  border-bottom: 1px solid rgba(255, 255, 255, 0.3);
}
.keys,
.top {
  overflow: hidden;
}
.keys span,
.top span.clear {
  float: left;
  cursor: pointer;
  width: 3em;
  height: 3em;
  line-height: 3em;
  color: #fff;
  text-align: center;
  user-select: none;
  transition: background-color 0.2s ease;
  border-right: 1px solid rgba(255, 255, 255, 0.3);
  border-bottom: 1px solid rgba(255, 255, 255, 0.3);
}
.keys span.operator {
  background: rgba(255, 255, 255, 0.2);
  border-right: 0;
  color: rgba(0, 0, 0, 0.5);
}
.keys span.equals {
  color: #888e5f;
}
.keys span:hover {
  background: rgba(255, 255, 255, 0.3);
  color: #FFF;
}
.keys span.equals:hover {
  background: rgba(255, 255, 255, 0.2);
  color: #fff;
}
.top span.clear:hover {
  background: rgba(255, 255, 255, 0.2);
  color: #FFF;
}