Where can I set the color of the multicolumn menu border?

If I use multicolumn menu, there's a white border between the columns, and I can't find how to set its color. (See the arrowed vertical borders in the picture.)

<off> BTW I didn't know TLB allowed such great eye-candy, the tool is way better than I thought when deciding to purchase it. Wonder why it's under-represented in GUI customization communities. </off>