9sq.css

:root {
    --bold-border-color: #ccc;
    --light-border-color: #555;
}

board {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    margin: 0 auto;
    width: calc(405px + 4px + 12px);
    border: 2px solid var(--bold-border-color);
}

panel {
    display: flex;
    justify-content: space-between;
    align-items: center;
    width: calc(405px + 4px + 12px);
    margin: 0 auto;
    margin-bottom: 2rem;
}

button-panel {
    display: grid;
    grid-gap: 0.5rem;

}

nine {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    border: 2px solid var(--bold-border-color);
}

sq {
    display: grid;
    align-items: center;
    justify-content: center;
    height: 45px;
    width: 45px;
    border: 1px solid var(--light-border-color);
    cursor: pointer;
}

sq.updating {
    background: #333;
}

sq.ctrl {
    border: 1px solid red;
}

sq.prefill {
    background: #222;
    cursor: default;
}

controls {
    display: grid;
    grid-template-columns: repeat(6, 1fr);
    width: calc(270px + 4px);
    border: 2px solid var(--bold-border-color);
}

ctrl {
    display: grid;
    align-items: center;
    justify-content: center;
    height: 45px;
    width: 45px;
    border: 1px solid var(--light-border-color);
    cursor: pointer;
}

ctrl.empty {
    cursor: default;
}

super-secret-stuff {
    display: block;
    height: 1px;
    width: 1px;
    overflow: hidden;
    opacity: 0;
}