The default behavior for box-sizing is content-box. This is applied when you deselect the rules in Chrome. Have you tried applying box-sizing: content-box in your own CSS file instead of inherit?
.ct-black-square
{
background-color: #88a0a9;
box-sizing: content-box;
-webkit-box-sizing: content-box;
-moz-box-sizing: content-box;
}
.ct-white-square
{
background-color: #fdfdfd;
box-sizing: content-box;
-webkit-box-sizing: content-box;
-moz-box-sizing: content-box;
}