style.css

:root {
    --pixel-size: 16;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
}

body {
    height: 100vh;
}

header {
    text-align: center;
    background-color: gainsboro;
}

footer {
    /* stick the footer to the bottom of the page */
    position: sticky;
    top: 100%;
    text-align: center;
    padding: 25px;
    margin-top: 30px;
    background-color: gainsboro;
}
footer a,
footer a:visited {
    font-style: italic;
    color: grey;
    text-decoration: none;
}
footer a:hover {
    color: rgb(78, 78, 78);
}

.settings-btn {
    width: 100px;
    height: 35px;
    margin: 20px 10px;
    border-radius: 10px;
    transition: transform 0.1s ease-in-out;
}
.settings-btn:hover {
    transform: scale(1.05);
}

.container {
    display: flex;
    width: 500px;
    height: 500px;
    margin: auto;
    border: 5px solid black;
    border-radius: 15px;
    margin-top: 2em;
    flex-wrap: wrap;
}

.grid-pixel {
    border: 0.05px solid black;
    width: calc(100% / var(--pixel-size));
}

button.active {
    background-color: black;
    color: white;
}