Change background for "white" border selection
I tend to invert colors for columns in Excel, means black background, white foreground.
When I set border color to white, borders "examples" in the formatting dialog cannot be seen, because the dialog background is white, too. This makes it hard to e.g. choose the thickness of the border AFTER you chose the color, and it is difficult to tell (except from the little buttons) which border is active and which not.
I suggest to darken the background in the dialog if "white" is choosen as border color or even use the background color of the cells, where the border will be applied.