Looks like this may in fact be an emacs issue (or, at least, an issue that is resolved by updating emacs), rather than a general gtk3+ theming one. Based on https://bugs.archlinux.org/task/48862, the following emacs commit fixes the missing icons: https://github.com/emacs-mirror/emacs/commit/3f4c6d52d345999938bc2d4a53246af4c61ef176
