style.css

@import url('https://fonts.googleapis.com/css?family=VT323');
html {
    background-color: rgb(60, 60, 65);
    height: 100%;
}

#title {
    text-align: center;
    font-size: 100px;
    margin: 20px;
    font-family: 'VT323', monospace;
    color: #fff;
}

.container {
    height: 550px;
    margin: 5% auto;
    position: relative;
    width: 680px;
    background-color: #333;
    margin-bottom: 0;
}

.result {
    background-color: #575757;
    height: 20%;
}

.calc-body #number-1,
#number-2 {
    width: 200px;
    height: 66px;
    padding: 50px;
    text-align: center;
    font-size: 50px;
}

.calc-body {
    width: 90%;
    margin: auto;
    margin-top: 20px;
}

.calc-body #multiply,
#division,
#subtract,
#sum {
    margin: 4%;
    margin-top: 50px;
    font-size: 90px;
    width: 100px;
    height: 100px;
}

#multiply:hover,
#division:hover,
#subtract:hover,
#sum:hover {
    border-radius: 20%;
    transition: all 0.5s;
    background-color: #9c9da0;
}

#show {
    text-align: center;
    font-size: 95px;
    font-family: sans-serif;
    color: white;
}