First off, write out the divs. You can use Emmet (in Codepen as standard, and at home if you install it) and write the following, then press tab
.board>div.square_row*8>.square*8
Next up, two normal bits of CSS, and two magic bits of CSS.
/* Normal bits */
.square_row{
overflow:hidden;
}
.square{
width:40px;
height:40px;
float:left;
background:#000;
}
/* Magic awesome bits */
.square_row:nth-child(odd) .square:nth-child(odd){
background:#eaeaea;
}
.square_row:nth-child(even) .square:nth-child(even){
background:#eaeaea;
}
So, basically, on all the odd rows, make the odd squares lighter, and on all the even rows, make the even squares lighter.
Done.
djave from eightarms.co.uk