style.css

/* Import Google font - Poppins */
@import url('https://fonts.googleapis.com/css2?family=Poppins:wght@400;500;600&display=swap');
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Poppins', sans-serif;
}

body {
    display: flex;
    align-items: center;
    justify-content: center;
    min-height: 100vh;
    background: #E3F2FD;
}
.flex {
    width: 50%;
    display: block;
    flex-direction: column;
    align-items: center;
    width: 750px;
}
.navbar {
    display: block  ;
    align-items: flex-start;
    text-decoration: underline;
    font-weight: 600;
}
.navbar a {
    color: #000;    
    padding-left: 10px;
    font-size: large;
    opacity: .6;
}

.navbar a:first-child {
    color: #000;
    padding-left: 20px;
}

.navbar a:hover {
    opacity: 1;
}

.main {
    width: 100%;
    padding: 35px 40px;
    border-radius: 20px;
    background: #141414;
    color: #B2B2B2;

    gap: 20px;
}

header {
    display: flex;
    align-items: center;
    justify-content: space-between;
}

header h2 {    
    font-size: 1.6rem;
    gap: 15px;
}

header .column {
    display: flex;
    align-items: center;
    
}

header span {
    font-weight: 500;
    margin-right: 15px;
    font-size: 1.19rem;
}

header input {
    outline: none;
    border-radius: 30px;
}

a {
    text-decoration: none;
}

a:after, a:before, a:default {
    text-decoration: none;
}

.volume-slider input {
    accent-color: #fff;
}

.keys-checkbox input {
    height: 30px;
    width: 60px;
    cursor: pointer;
    appearance: none;
    position: relative;
    background: #4B4B4B
}

.keys-checkbox input::before {
    content: "";
    position: absolute;
    top: 50%;
    left: 5px;
    width: 20px;
    height: 20px;
    border-radius: 50%;
    background: #8c8c8c;
    transform: translateY(-50%);
    transition: all 0.3s ease;
}

.keys-checkbox input:checked::before {
    left: 35px;
    background: #fff;
}

.piano-keys {
    display: flex;
    list-style: none;
    margin-top: 40px;
}

.piano-keys .key {
    cursor: pointer;
    user-select: none;
    position: relative;
    text-transform: uppercase;
}

.piano-keys .black {
    z-index: 2;
    width: 44px;
    height: 140px;
    margin: 0 -22px 0 -22px;
    border-radius: 0 0 5px 5px;
    background: linear-gradient(#333, #000);
}

.piano-keys .black.active {
    box-shadow: inset -5px -10px 10px rgba(255,255,255,0.1);
    background:linear-gradient(to bottom, #000, #434343);
}

.piano-keys .white {
    height: 230px;
    width: 70px;
    border-radius: 8px;
    border: 1px solid #000;
    background: linear-gradient(#fff 96%, #eee 4%);
}

.piano-keys .white.active {
    box-shadow: inset -5px 5px 20px rgba(0,0,0,0.2);
    background:linear-gradient(to bottom, #fff 0%, #eee 100%);
}

.piano-keys .key span {
    position: absolute;
    bottom: 20px;
    width: 100%;
    color: #A2A2A2;
    font-size: 1.13rem;
    text-align: center;
}

.piano-keys .key.hide span {
    display: none;
}

.piano-keys .black span {
    bottom: 13px;
    color: #888888;
}

.box {
    width: 150px;
    height: 50px;
    color: #fff;
    display: flex;
    justify-content: center;
    align-items: center;
    margin: 10px;
    text-decoration: none;
}

.box .rectangle {
    display: flex;
    justify-content: center;
    align-items: center;
    color: #fff;
    width: 100%;
    height: 100%;
    padding: 10px;
    border-radius: 40px;
    border: 4px solid #fff;
    text-decoration: none;
}

@media screen and (max-width: 815px) {
    .wrapper {
        padding: 25px;
    }
    header {
        flex-direction: column;
    }
    header :where(h2, .column) {
        margin-bottom: 13px;
    }
    .volume-slider input {
        max-width: 100px;
    }
    .piano-keys {
        margin-top: 20px;
    }
    .piano-keys .key:where(:nth-child(9), :nth-child(10)) {
        display: none;
    }
    .piano-keys .black {
        height: 100px;
        width: 40px;
        margin: 0 -20px 0 -20px;
    }
    .piano-keys .white {
        height: 180px;
        width: 60px;
    }
}

@media screen and (max-width: 615px) {
    .piano-keys .key:nth-child(13),
    .piano-keys .key:nth-child(14),
    .piano-keys .key:nth-child(15),
    .piano-keys .key:nth-child(16),
    .piano-keys .key :nth-child(17) {
        display: none;
    }
    .piano-keys .white {
        width: 50px;
    }
}