styles.css

body {
  font-size: 2.5vh;
  top: 15%;
  left: 40%;
  margin: 0 auto;
  background-color: black;
  font-family: 'Roboto', sans-serif;
}

h1 {
  color: white;
  text-align: center;
  margin: 0;
}

#calculator {
  width: auto;
  height: auto;
  padding: 1.5vh;
  border: 5px solid #666666;
  border-radius: 15px;
  margin: 0;
  position: absolute;
  top: 50%;
  left: 50%;
  -ms-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
}

.box {
  text-align:right;
  font-size: 2.8vh;
  height: 3vh;
  line-height: 3vh;
  padding: 1em;
  border: 2px solid #e68a00;
  color: white;
  border-radius: 15px;
  background-color: black;
}

#last_operation_history {
  height: 1.8vh;
  font-size: 1.8vh;
  text-align:right;
  padding-right: 1.5vh;
  color: rgb(177, 176, 176);
  background-color: black;
}

button {
  width:9vh;
  height: 9vh;
  font-weight:bold;
  font-size: 3vh;
  color: white;
  background-color: #666666;
  border-radius: 50%;
  border: 1px solid #333333;
}

button:hover {
  background-color: #999999;
}

.operator {
  background-color: #e68a00;
}

.operator:hover {
  background-color: #ffd11a;
}

#equal_sign {
  width: 100%;
  border-radius: 15px;
}