updated-piano.css

html {
    box-sizing: border-box;
}

*,
::after,
::before {
    box-sizing: inherit;
}

#piano {
    width: 1100px;
    height: 400px;
    background: linear-gradient(#740404, #033d03, #5a055a, #520303);
    background: -webkit-linear-gradient(#740404, #033d03, #5a055a, #520303);
    margin: 80px auto;
    position: relative;
    border-radius: 30px;
}

.keys {
    background-color: #220a0a;
    width: 980px;
    height: 290px;
    padding-left: 5px;
    margin: 60px auto;
    overflow: hidden;
    border-radius: 0 0 20px 20px;
}

.key {
    background-color: rgb(115, 185, 247);
    width: 40px;
    height: 280px;
    float: left;
    margin: 3px;
    position: relative;
    border-radius: 0 0 8px 8px;
}

.key.black-key::after {
    content: "";
    position: absolute;
    left: -20px;
    width: 36px;
    height: 180px;
    background-color: #060606;
    border-radius: 0 0 6px 6px;
}

@media (max-width: 768px) {
    #piano {
        width: 358px;
    }

    .keys {
        width: 318px;
    }

    .logo {
        width: 150px;
    }
}

@media (max-width: 1199px) and (min-width: 769px) {
    #piano {
        width: 675px;
    }

    .keys {
        width: 633px;
    }
}