Today I figured out how to get GTK engines to work on my amd64 machine when using programs like
mozilla-firefox-bin which are pre-compiled for x86. By "figured out" of course I mean "read some ebuilds someone else wrote and then modified them to work for themes of my choosing".
Basically you have to find a binary pre-compiled x86 version of the theme and unpack it into
/emul/linux/x86/usr/lib/gtk-2.0/2.4.0/engines/. The theme I was after is Rezlooks. Ubuntu seems like a good place to find pre-compiled versions of themes. In fact you can download a .deb for Rezlooks here. (You do have to register at the forums to download the file, which is annoying.) Download and unpack
/usr/portage/distfiles, then run this ebuild. (Disclaimer: I have no idea what I'm doing. If the ebuild deletes your boot partition and kicks your cat, don't blame me.)
I need to figure out how to set up a chroot to compile things for x86 myself, probably. Another day.