style.css

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

body {
    background-color: skyblue;
    font-family: 'Open Sans', sans-serif;
}

.background {
    background-color: lightgray;
    margin: 60px auto;
    display: flex;
    align-items: center;
    justify-content: center;
    height: 550px;
    width: 320px;
    border :10px solid black;
    border-radius: 7px;
}

#result {
    height: 120px;
}

#keyboard {
    height: 400px;
}

#history {
    text-align: right;
    height: 20px;
    margin: 0 20px;
    padding-top: 20px;
    font-size: 15px;
    color: #919191;
}

#output {
    background-color: lavender;
    text-align: right;
    height: 60px;
    margin: 10px 20px;
    font-size: 30px;
}

.operator,
.number,
.empty {
    width: 50px;
    height: 50px;
    margin: 15px;
    float: left;
    border-radius: 50%;
    border-width: 0;
    font-size: 15px;
}

.number,
.empty {
    background-color: #eaedef;
}

.number,
.operator {
    cursor: pointer;
}

button:nth-child(4) {
    font-size: 20px;
    background-color: #20b2aa;
}

button:nth-child(8) {
    font-size: 20px;
    background-color: #ffa500;
}

button:nth-child(12) {
    font-size: 20px;
    background-color: #f08080;
}

button:nth-child(16) {
    font-size: 20px;
    background-color: #7d93e0;
}

button:nth-child(20) {
    font-size: 20px;
    background-color: #9477af;
}

.operator:active,
.number:active {
    font-size: 13px;
}

.operator:focus,
.number:focus,
.empty:focus {
    outline: 0;
}