styles.css

body {
  width: 502px;
  margin: 20px auto;
  font-family: 'Source Sans Pro', sans-serif;
  letter-spacing: 5px;
  font-size: 1.8rem;
  -moz-user-select: none;
  -webkit-user-select: none;
  -ms-user-select: none;
}

.calculator {
  padding: 20px;
  -webkit-box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  border-radius: 1px;
  background-color: black;
}

.display {
  background-color: white;
  border: 1px solid #ddd;
  border-radius: 1px;
  height: 145px;
  padding-right: 15px;
  text-align: right;
  font-size: 2.5rem;
  overflow-x: auto;
  transition: all .2s ease-in-out;
  margin-bottom: 10px;
}

.display:hover {
  border: 1px solid #bbb;
  -webkit-box-shadow: inset 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: inset 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
}

#equation {
  height: 45px;
  font-size: 100%;
  margin: 10px;
}

#output {
  height: 60px;
  font-size: 75%;
  margin: 10px;
}

.operators div {
  display: inline-block;
  border: 1px solid #bbb;
  border-radius: 1px;
  width: 83px;
  text-align: center;
  padding: 10px;
  margin: 20px 0px 10px 0;
  cursor: pointer;
  background-color: #ddd;
  transition: border-color .2s ease-in-out, background-color .2s, box-shadow .2s;
}

.operators div:hover {
  background-color: #ddd;
  -webkit-box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  border-color: #aaa;
}

.operators div:active {
  font-weight: bold;
}

.leftPanel {
  display: inline-block;
}

.button {
  display: inline-block;
  border: 1px solid #ddd;
  border-radius: 1px;
  height: 72px;
  width: 105px;
  text-align: center;
  font-size: 2.5rem;
  padding: 10px;
  margin: 10px 0px 10px 0;
  cursor: pointer;
  background-color: white;
  transition: border-color .2s ease-in-out, background-color .2s, box-shadow .2s;
}

.button:hover {
  background-color: #f1f1f1;
  -webkit-box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  border-color: #bbb;
}

.numbers div:active {
  font-weight: bold;
}

.equal {
  display: inline-block;
  border: 1px solid grey;
  border-radius: 1px;
  width: 105px;
  height: 348px;
  text-align: center;
  font-size: 2.5rem;
  padding: 127px 10px;
  margin: 10px 0px 10px 0;
  vertical-align: top;
  cursor: pointer;
  color: black;
  background-color: light grey;
  transition: border-color .2s ease-in-out, background-color .2s, box-shadow .2s;;
}

.equal:hover {
  background-color: #f1f1f1;
  -webkit-box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  border-color: #bbb;
}

.equal:active {
  font-weight: bold;
}