style.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Courier New', Courier, monospace;
    background-color: lightskyblue;
}

/* Containers */
#windowContainer {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 100vw;
    height: 100vh;
    background-color: red;
}

#uiContainer {
    display: flex;
    align-items: center;
    flex-direction: column;
    width: 800px;
    height: calc(100% - 50px);
    background-color: yellow;
}

/* Piano */
#uiHeader {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 100%;
    height: 50px;
    background-color: green;
}

#piano {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 50%;
    height: 50%;
    background-color: pink;
}

.pianoKey {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 50px;
    height: 200px;
    background-color: white;
    border: 1px solid black;
}

.pianoKey:hover {
    background-color: lightgray;
}