styles.css

html{
    height: 100%;
    background-color: #f6f6f6;
    font-family: Arial, Helvetica, sans-serif;
}

body{
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100%;
    background-color: white;
    border: 1px solid purple;
}

#main{
    background-color: #aaaaaa;
    box-sizing: border-box;
    height: 500px;
    width: 400px;
    border-radius: 20px;
    border: 5px solid black;
    padding: 20px;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
}

#screen{
    box-sizing: border-box;
    height: 110px;
    width: 100%;
    background-color: #eeeeee;
    border-radius: 10px;
    border: 2px solid black;
    padding: 10px;

    display: flex;
    flex-direction: column;
    justify-content: space-between;
    align-items: end;
}

#screenTop{
    font-size: 1.8em;
}

#screenBottom{
    box-sizing: border-box;
    font-size: 3.5em;
    line-height: 0.9em;
}

.btn-lg{
    width: 48%;
    text-align: center;
}

.btn-sm{
    width: 22%;
}

.btn{
    height: 50px;
    display: flex;
    justify-content: center;
    align-items: center;
    border-radius: 5px;
    font-size: 1.8em;
}

#btn-area{
    box-sizing: border-box;
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    align-items: space-between;
    height: 320px;
    padding-bottom: 10px;
}