As I understood it, GIMP moved to Gtk+ 3.0 in the version 2.10. From what I've read, this makes GIMP support theming. But, after looking around, no one have made any GIMP themes even though there's this new functionality. Back in 2.8 where they don't officially support "themes", people still make some "themes" for GIMP.
Anyone knows why?
No one can be bothered making Gtk+ themes for GIMP?
You'd better check the official roadmap rather than old news