Here is the difference between icons and other UI elements between retina and not-retina screen: It's not a pressing matter but it makes the UI unreadable a bit ;-)
Hm, strange. I have Retina MBP + old non-retina 24' Apple Cinema Display. I guess it's a rare combo^^
@Kamil Kucma is it displaying incorrectly on both your screens? or just the external (which is what I suspect)?
OK, it must be using the primary screen to detect retina and assuming all additional screens are the same... I'll notify the team and we if they can improve the support. If you can please open a ticket with support@buildbox.com with the info contained in this thread. I have retina Macbook Pro and an older screen here (not currently connected) so I may be able to duplicate the issue in house.