.number-grid-cell {
    display: inline-block;
    width: 40px;
    /* or any size */
    height: 40px;
    /* or any size */
    border-radius: 50%;
    /* or any style */
    text-align: center;
    line-height: 40px;
    font-size: clamp(1em, 1.6em, 22px);
    text-align: center;
}

.number-grid-cell.selected{
    background-color: green;
    color: white;
}