That entry is present and enabled.
(Otherwise it also wouldn't show up in 2.5 would it?)
Just had a look at your updated skin in 3.0 and there is a faint divider line (colour:2,13,12) between the column browser columns.
EDIT:
also, if you don't want the top border to match the panel border, you can override it with:
<element id="MainPanelTopBorderColour" bdr="54,62,69" />
I believe that is this line, but that is not related to the issue I am raising? Or am I missing something?
so if you wanted that red line under the tabs but wanted the divider border to stay "2,13,12", you would need:
<element id="MainPanelTopBorder">true</element>
<element id="Panel.Body.Default" bg="16,19,16" fg="127,132,133" bdr="2,13,12"/>
<element id="MainPanelTopBorderColour" bdr="255,0,0" />
otherwise the red border would also be "2,13,12"