styles.css

* {
    box-sizing: border-box;
}

body {
    margin: 0;
    padding: 0;
}

img {
    width: 100%;
}

.container {
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    gap: 1rem;
    min-height: 100vh;
    padding: 0 1rem;
    background-image: url(images/rock1.png), linear-gradient(90deg, red, blue);
    background-size: cover;
    background-position: center;
    background-blend-mode: multiply;
}

.keys {
    display: flex;
    justify-content: center;
    align-items: center;
    gap: 1rem;
}

.key {
    width: 75px;
    height: 75px;
    border: 2px #252525 solid;
    font-size: 2rem;
    text-align: center;
    color: #252525;
    padding-top: 1.5rem;
    border-radius: 10px;
    transition: all .07s;
}

.playing {
    transform: scale(1.1);
    border-color: #ffc600;
    box-shadow: 0 0 10px #ffc600
}

.sound {
    color: #ffc600;
}

.key:hover,
.key:focus {
    border: 5px transparent solid;
    border-image: linear-gradient(220deg, red, blue) 1;

}

@media screen and (min-width: 1028px) {

    .container {
        flex-direction: row;
    }

    .key {
        width: 100px;
        height: 100px;
    }
}