design.css

body{
    padding: 0;
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    min-height: 100vh;
    background: rgb(231, 237, 248);
}

.container{
    width: 60vh;
    height: 60vh;
    padding: 10px;
    border-radius: 10px;
    box-shadow: 10px 10px 15px rgb(193, 211, 243),
                -10px -10px 15px rgb(201, 218, 248);
}

.display #output{
    width: 60vh;
    height: 8vh;
    font-size: 30px;
    border: none;
    border-radius: 8px;
    box-shadow: inset 4px 4px 10px rgb(159, 193, 246),
                inset -4px -4px 10px rgb(215, 227, 248);
    text-align: right;
}

.buttons{
    width: 100%;
    display: table;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    margin-top: 2em;
}

.buttons .row .button{
    height: 55px;
    width: 55px;
    margin: 10px;
    font-size: 15px;
    background: #e9edf5;
    border: none;
    border-radius: 45px;
    box-shadow: 7px 5px 10px rgb(174, 194, 227),
                -5px -5px 10px rgb(215, 227, 248);
}