style.css

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

html {
  font-family: "Roboto", "Noto Sans", "Helvetica", sans-serif;
  font-size: 14px;
  background-color: #3498DB;
  height: 100%;
}

.calculator {
  width: 350px;
  height: auto;
  margin: 70px auto 0;
  overflow: hidden;
  box-shadow: 4px 4px rgba(0, 0, 0, 0.2);
}

.calculator span {
  -moz-user-select: none;
  user-select: none;
}

.top {
  position: relative;
  height: 150px;
  background-color: white;
}

.top .unit {
  text-transform: uppercase;
  position: absolute;
  top: 10px;
  left: 10px;
  font-weight: 700;
  color: #757575;
}

.top .screen {
  position: relative;
  width: 100%;
  top: 20%;
  height: 80%;
}

.screen > div  {
  display: flex;
  align-items: center;
  justify-content: flex-end;
  width: 100%;
  padding: 5px;
  text-align: right;
}

.screen .input {
  color: #757575;
  height: 60%;
  font-size: 35px;
}

.screen .result {
  color: #9e9e9e;
  font-size: 20px;
  height: 40%;
}

.bottom {
  background-color: #2D2D2D;
  height: 300px;
  color: #fff;
  cursor: pointer;
}

.bottom section {
  position: relative;
  height: 100%;
  float: left;
  display: flex;
}

.keys {
  width: 80%;
}


.keys .column {
  display: flex;
  flex-grow: 1;
}

.keys .column, .operators {
  flex-direction: column;
  justify-content: center;
  align-items: center;
}

.keys .column span, .operators span {
  position: relative;
  overflow: hidden;
  flex-grow: 1;
  width: 100%;
  line-height: 1;
  padding: 10px;
  display: flex;
  justify-content: center;
  align-items: center;
  transition: background-color 0.5s;
}

.keys .column span {
  font-size: 25px;
}

.keys .column span:hover, .operators span:hover {
  background-color: rgba(0, 0, 0, 0.2);
}

.operators {
  width: 20%;
  font-size: 18px;
  background-color: #434343;
}

.delete {
  font-size: 16px;
  text-transform: uppercase;
}

.credit {
  display: block;
  text-align: center;
  width: 100%;
  position: absolute;
  bottom: 20px;
  margin: 0 auto;
}

.credit a, .error {
  color: #C2185B;
}