index.css

* {
    font-family: 'Courier New', Courier, monospace;
}
body {
    margin: 2rem;
    text-align: center;
}
div, textarea, button {
    font-size: 2rem;
}
div {
    margin: 0.2rem 0;
}
textarea, button {
    background-color: lightgray;
    border-radius: 1em;
    border: 0.5rem solid gray;
}
textarea {
    text-align: center;
    letter-spacing: 0.3em;
    padding: 1em;
    box-sizing: border-box; 
    max-width: 100%;
    max-height: 100vw;
}
button {
    background-color: gray;
    color: white;
    font-size: 3rem;
    padding: 0 0.5em;
    cursor: pointer;
}

table {
    margin-top: 1rem;
    border-collapse: collapse;
    border: 0.5rem solid gray;
}
td {
    width: 1rem;
    height: 1rem;
    border: 1px solid blue;
}
td.on {
    background-color: navy;
}

@media (max-width: 650px) {
    body {
        margin: 0.5rem;
    } 
}

@media (max-width: 300px) {
    body {
        margin: 0.2rem;
    } 
    textarea {
        font-size: 1.2rem;
    }
    button {
        font-size: 2rem;
    }
}