style.css

body{
    background-color: black;
}
#container{
    width: 100%;
    height: 550px;
}
#calculator{
    width: 310px;
    height: 520px;
    background-color: #efefef;
    margin: 0 auto;
    position: relative;
    margin-top: 70px;
}
.number, .operator, .empty{
    width: 50px;
    height: 50px;
    border-radius: 50%;
    margin: 10px;
    border-style: none;
    font-weight: bold;
}
.number, .operator{
    cursor: pointer;
}
.number:hover{
    border-radius: 50%;
    background-color: lightgray;
}
button:nth-child(4){
    background-color:chocolate;
    font-size: 15px;
}
button:nth-child(8){
    background-color:cornflowerblue;
    font-size: 15px;
}
button:nth-child(12){
    background-color:darkgreen;
    font-size: 15px;
}
button:nth-child(16){
    background-color:indigo;
    font-size: 15px;
}
button:nth-child(20){
    background-color:yellow;
    font-size: 15px;
}
#result{
    height: 120px;
}
#history{
    text-align: right;
    height: 20px;
    font-size: 15px;
    font-weight: lighter;
    padding-top: 15px;
    margin: 10px;
}
#output{
    text-align: right;
    height: 100px;
    font-size: 30px;
    font-weight: bolder;
    margin: 15px;
}