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.)

Oops I forgot to to skin these elements :oops: You are first who noticed this. In the next version I'll add these lines into the skinning. Thank you.

Wonder why it's under-represented in GUI customization communities.

I try to promote TLB there, but maybe not very successful. :oops: