Now in 5.33pre8, there's some basic macOS Retina support -- note that it is only supporting Retina for the track panels, mixer, transport, toolbars, and tab controls. The arrange view and MIDI editor are still rendered at low resolution.
Here is the theme to use for macOS with retina displays:
http://landoleet.org/dev/Default_5.0...ReaperThemeZip
and use 5.33pre8 for macOS (which will be up shortly).