styles.css

* {
    margin: 0;
    padding: 0;
    font-family: 'ZCOOL KuaiLe', cursive;
}

body {
    background-color: #FFFFFF;
    min-height: 100%;
}

html,body {
    overflow: auto;
}

header {
    margin: 20px;
    font-size: 1.5em;
}

h1 {
    text-align: center;
    color: #fcba04;
}

#content {
    width: 100%;
}

#controls {
    text-align: center;
    margin: 10px auto 20px auto;
    width: 90vw;
    max-width: 800px;
    height: 50px;
    display: flex;
    justify-content: center;
    
}

#controls button {
    margin: 0 2px;
    color: #a50104;
    font-size: 1.5em;
    border-radius: 5px;
}

#clear {
    flex-grow: 1;
}

#sub {
    flex-grow: 1;
}

#test {
    flex-grow: 2;
}

 main {
    display: flex;
    flex-direction: column;
    align-items: center;
    margin: auto;
    padding: 0 5px;
}

#puzzle {
    width: 90vw;
    height: 90vw;
    max-width: 800px;
    max-height: 800px;
    margin: auto;

}

table {
    margin: auto;
    background-color: #07B2FF;
    border-collapse: collapse;
    border-radius: 2px;
    width: 100%;
    height: 100%;
    table-layout: fixed;
}

td {
    border: 1px solid black;
    width: 10%;
    height: 10%;
    text-align: center;
    font-size: 4vw;
    -moz-user-select: none;
    user-select: none;
    font-family: 'Lato', sans-serif;

}

td:nth-child(3), td:nth-child(6)  {
    border-right: 3px solid black;
}

tr:nth-child(3), tr:nth-child(6) {
    border-bottom: 3px solid black;
}

.immutable {
    color: #F2F2F2;
}

@media (min-width: 1300px) {

    td {
        font-size: 40px;
    }

}

@media (min-width: 750px) and (max-width: 1400px) {

    #puzzle {
        width: 70vmin;
        height: 70vmin;
    }

    #controls {
        width: 70vmin;
    }
}