A lot of work went into making 2018r3 to making it look better in Mojave. There were a lot of things that didnt matter when everything was light which do matter now so the colors of the old IDEs may be a little off.
If you open the System Preferences what are your settings for Accent and Highlight colors? Changing these affects the normal system colors which Xojo uses for various things like selection and row highlights.
At this point I will assume it is an incompatibilty between Mojave and Xojo2016, and realize it is what it is…
Upon further investigation… I found if I changed the ACCENT COLOR in Preferences, the “purple” changed to what ever color I had selected… However… it would NOT change back to BLUE… it remained which ever other color had been the last one selected.
And I had selected purple as a test… not realizing it wouldn’t change back.
this has nothing to do with hardcoded colors…the colors DO change… it just cannot be changed back to BLUE once it has been changed to something else… I understand it to be an anomoly in 2016, and will live with it until I upgrade to 2018/19