Thanks for the heads up)
When using latest Reaper Dev-version, the play-head, splash and most elements seems to display correct here. Some 3rd. party GUI are totally blacked out though…
I believe the devs are still working on adjustments to apple metal framework and Retina support.
Btw. for Mojave I recommend NightOwl, for easy dark/light mode switching with a shortcut.
https://nightowl.kramser.xyz/