style.css

body {
    text-align: center;
}

#frame {
    display: inline-block;
    background-color: #1C262F;
    margin-top: 15px;
    border-radius: 15px;
    box-shadow: 3px 3px 5px black;
    box-shadow: inset 0px 0px 8px white;
    padding: 30px 30px;
}

#frame-top {
    display: inline-block;
    background-color: #EBE7E6;
    height: 180px;
    width: 300px;
    box-shadow: inset 0px 0px 3px black;
    font-family: 'Cinzel', serif;
    font-size: 13px;
}

#frame-top h1 {
    display: inline-block;
    font-family: 'Oswald', sans-serif;
    font-weight: 200;
    font-size: 30px;
    margin: 20px 0px 0px 0px;
}

#frame-top h1 span{
    font-weight: 700;
}

#screen {
    display: inline-block;
    background-color: #64786C;
    height: 80px;
    width: 250px;
    margin: 10px auto;
    box-shadow: inset 0px 0px 7px black;
}

#screen p {
    font-family: 'Rationale', sans-serif;
    font-size: 30px;
    margin: 3px 10px;
    text-align: right;
}

#advanced-buttons {
    display: block;
    margin-top: 15px;
}

.advanced {
    background-color: hsl(223, 28%, 8%);
    color: white;
    font-family: 'Oswald', sans-serif;
    font-weight: 400;
    font-size: 16px;
    border-radius: 4px;
    border: 0px solid black;
    box-shadow: 2px 2px 0px hsl(223, 28%, 4%);
    width: 50px;
    height: 25px;
    padding: 0px;
    margin: 3px 3px;
}

#digit-buttons .advanced {
    margin: 3px 3px 15px 3px;
}

#digit-buttons {
    display: inline-block;
    width: 180px;
    margin-top: 6px;
}

.digits {
    background-color: #e3e6e8;
    color: black;
    border-radius: 6px;
    border: 0px;
    margin: 3px 3px;
    box-shadow: 2px 2px 0px #abb4ba;
    font-family: 'Ubuntu', sans-serif;
    font-size: 22px;
    width: 50px;
    height: 40px;
}

#zero {
    width: 109px;
}

#operator-buttons {
    display: inline-block;
    width: 60px;
}

.operators {
    background-color: #536693;
    color: white;
    border-radius: 6px;
    border: 0px;
    margin: 3px 3px;
    box-shadow: 2px 2px 0px #374462;
    font-family: 'Ubuntu', sans-serif;
    font-size: 22px;
    width: 50px;
    height: 40px;
}