I know this is going to sound strange, but I started seeing this in the last version, and I am seeing it also in 2013.2.918.commercial
I use the MetroBlack Theme, and for Grids (and also see my related note about DatePickers) I am seeing a white line as the border between columns for my Grid. This is only the case in Chrome v29. I am also attaching screen shots of the same app running in FireFox and IE11. They are displaying fine.
See My related post with the same problem with the DatePicker