So finally, Kame Editor has reached v1.0.0 and can be used to create full featured 3DS themes. You can find the Mac builds here: https://bitbucket.org/beelzy/kame-editor/downloads/ I have a PKGBUILD in the AUR for Archlinux users here: https://aur.archlinux.org/packages/kame-editor-git And you can find the source code here: https://gitlab.com/beelzy/kame-editor
If you are or know a distro maintainer who would be interested in packaging this app, there are packaging notes in the source repo, but it shouldn't be too complicated.
There will be more to come; currently, Kame Editor doesn't display the 3D folder in the top screen. I'm still trying to decide how best to do that.
Theoretically, Kame Editor should work on Windows; I've just never tried it and wasn't sure if there was interest for it.