Adding Class to 3 Column Layout Elements is Screwing up Cont

I'm wanting to use Sitefinity's 3 column layout element. I set the widths and the spacing and want a 1px border between each column, so I add a style class to the center column (.columnCenter) that says --

.columnCenter border-left:1px dashed #BCBEC0; border-right:1px dashed #BCBEC0;

Well the extra 2 pixels are forcing the last column below the others.

Is there a way to apply custom widths to the layout elements? I don't understand why that's not an option...