.word-puzzle-container, .numbers-sums-container {
    display: grid;
}

.grid-cell {
    display: inline-block;
    width: 40px;
    /* or any size */
    height: 40px;
    /* or any size */
    border: 1px solid #ccc;
    /* or any style */
    text-align: center;
    line-height: 30px;
    /* match height for vertical center */
}

.grid-cell.selected {
    background-color: green;
    border: 2px solid green;
}

/* .grid-cell.certain {
    color: yellow;
} */