SOLVED: "Unable to locate theme engine in module_path"

[JohnN]

At some recent point, it seems that portage stopped pulling in x11-themes/gnome-themes-standard with gtk+ and applications that depend on these themes? I suddenly lost buttons and menus in lots of applications until I found this suggestion on a Ubuntu forum: http://askubuntu.com/questions/774664/gtk-warning-unable-to-locate-theme-engine-in-module-path-adwaita-error-o. I installed the Gentoo package of the same name, adding it to world, for now, and my problems all disappeared. (Well, not all of them, but... )