v5.34pre releases include experimental functionality for the first steps towards Windows 8.1+ HiDPI support, and MacOS Retina support in Reaper.
Here is a rebuild of the Default 5.0 theme, for testing with that functionality.
edit: updated on Feb 15th 2017
Test with 5.34pre7 or later!
http://landoleet.org/dev/Default_5.0...ReaperThemeZip
- Install the theme.
- On Windows: Options > Preferences > General > Advanced UI/system tweaks > HiDPI mode. Restart REAPER.
- Options > Themes > select Default_5.0_hidpi.
Its currently just the standard sized layouts, and has been produced for 200% scaling. Quality of appearance at other scales may or may not appear aggreeable to you. Both theme and the HiDPI functionality are very much a work in progress, but this is a good point to start testing. You'll find many things aren't correct yet, and while its probable that we know about many of them, please don't assume so; if its wrong then please do report it, there
will be others
re: 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.