body, html {width: 100%; height: 100%; overflow: hidden;}

.wrapper {width: 70vmin; height: 70vmin; margin: 0 auto; background-color: #AAACA8;}
.grid {width: 100%; position: relative;}
.cell {
    width: 15vmin;
    height: 15vmin;
    margin: 2vmin 2vmin 0 0;
    background-color: #C5C6BE;
    float: left;
}
/* add left margin to first childs of rows */
.cell:nth-child(4n+1) {
    margin-left: 2vmin;
}

.number {
    position: absolute;
    height: 15vmin;
    width: 15vmin;
    font-size: 5vmin;
    font-weight: bold;
    text-align: center;
    line-height: 15vmin;
    background-color: #F44336;
    color: #FFFFFF;
    transition: all .3s;
}