Which "thin inside line"? Do you mean the border to the left of the categories box, where it is doubled as compared to the right sideboxes? This disparity is caused partly by the sideboxes having borders all around, and partly by the sidebox width being set in admin to the same width as the side columns, with borders added.
Find/add in your stylesheet
Code:
.leftBoxContainer, .rightBoxContainer {
border: 1px solid #8E0EBD;
margin-top: 1.5em;
width: 159px !important;
}
.leftBoxContainer {border-left: none;}
.rightBoxContainer {border-right: none;}
Better would be to set the Column Left/Right Boxes width to 159px in admin > Configuration > Layout Settings and remove that from the stylesheet.
If you want something else, please explain in more detail. I suppose the problem you saw was corrected by the time I saw it...