People are working on HiDPI toolbar bitmaps, but the LibreOffice code still needs to support to loading them.
The current code just doubles all the bitmaps: http://opengrok.libreoffice.org/xref/core/vcl/source/window/toolbox2.cxx#1112
This is a nice little hack that can stay around as long as we still have some themes without better bitmaps, but one day someone needs to write a bit of code that fixes (some of) the callers of this routine, and make them load larger bitmaps if they exist for a theme.
One open issue is whether it worth it using the SVG format.
I believe this is an interesting Easy Hack that could make a nice visual improvement on these pretty screens. For now, it would probably only apply to Linux and Windows, and somewhat different code would be needed for the Mac.
Migrating Whiteboard tags to Keywords: (EasyHack DifficultyInteresting SkillCpp TopicUI)
JanI is default CC for Easy Hacks (Add Jan; remove LibreOffice Dev List from CC)
I saw this was fixed in 5.3.