I haven't written any documentation on the HiDPI functionality because I don't have a definitive understanding of
exactly what it is doing in detail and I don't want to publish wrong information; during pre-release things were done in numerous different ways, and at least one important implementation method has even changed since Reaper V6 was released. The HiDPI functionality is not currently described in the theming SDK.
I gladly offer assistance here in the forum, I have briefly explained how to enact a HiDPI theme
HERE, and in the V6 theme I kept the HiDPI sections of the code separate so they could be used as exemplars, though I acknowledge that might be of limited use because they are wrapped up in lots of complex macros. So ..er.. ask away, and I'll tell you what I'm
pretty sure is happening, just don't ask me to sign any paperwork
The image based approach that is used it burdensome on the themer, but it and the simple WALTER layout code method means that no experienced themer should find it technically challenging. If you, or anyone else, is interested in making a HiDPI theme then please let me know, because there's unlikely to be effort expended on documentation if there's no demand - all I've seen so far is modders
removing HiDPI functionality
There also appears to be an understanding gap between theme modders and the way HiDPI works at a hardware/OS level (see
this thread), the state of HiDPI implementations in OSs is still changing, and the hardware landscape is also changing in time as we gradually see less of the really silly annoying things like 1080 @ 133% laptops. So its, y'know, complicated.
The only other thing the V6 default theme has that is new is the theme adjuster script control functionality, see: Adjusting themes using scripting :
HERE. Adding theme adjuster functionality to a theme without doing any scripting :
HERE. Though I've retro-fitted that to the Default 5 theme now, so I'm not sure it counts as V6 new any more!