I have seen my library being mentioned here multiple times, I am glad it is of use. While GTK4 still allows to set that preference in other ways, for example it tells me that it picked up mine from XSettings, I have added support for setting it from that environment variable in GTK4 too. I also added another option, to overwrite what the application has set, to the user preference (which was never possible with plain GTK versions). It will be part of next release (once I finish the global menu part), which will be picked up by Debian at some point, or you can self build it.