This will make any white box preceded by br
and a gray box will have a (black) border on top of it.
.mbaBox+br+.mbaBox-white {
position: relative;
}
.mbaBox+br+.mbaBox-white:after {
content: " ";
background: #000; /* your color */
width: 100%;
height: 1px;
position: absolute;
top: -10px;
left: 0;
}