Sometime in the last few releases - maybe sometime in the last few months - the vertical bars which separate the columns in the column headers have become almost invisible. This happens with most skins. Screen shots showing the problem with 4 different skins are attached. It looks to me like the column separator is one pixel wide, and it's almost the same color as the background of the column header. That's pretty much invisible on a 5K monitor.
The reason this is a problem is that to adjust column widths, you have to grab one of these column separators. If I can't see them I can't grab them.
Of course this might well be a case of my eyesight getting weaker. :-) But it would be cool if these separators were more visible.
MC 28.0.79. macOS 12.0.1. 2017 27-inch iMac with Retina 5K display (5120 x 2880).