I've been having this non-critical issue with banshee for some time, and finally got around to figuring out what was causing it. I'm using KDE4, and with my normal account, banshee had some missing icons in the left-hand nav: Music, Videos, Now Playing... possibly some others, but many icons were not missing. I have a second account that I use for debugging problems like this, and while trying to debug a whole separate problem, I discovered that the icons were NOT missing in the second account. So I systematically tested every configuration file in my home directory, and discovered that .gtkrc-2.0 was responsible. Looking at that file, it seems that it specifies oxygen-gtk as a gtk-theme-name. So I installed x11-themes/oxygen-gtk, and all was well. I've sinced looked at my system settings, and oxygen-gtk was in fact selected as the GTK theme, but I never actually selected this, and it also happens to be selected for my second account, although the .gtkrc-2.0 file didn't exist until after I checked this for this user (so, checking System Settings -> Application Appearance -> Gtk Config caused .gtkrc-2.0 to be written). So my suggestion: make Gtk-dependent applications (like banshee) depend on oxygen-gtk. Or make KDE depend on oxygen-gtk, since this theme seems to be selected by default.
not going to add such depend on random apps, so moving to kde maintainers ->
I have very strong doubts about this. Additional themes should stay additional. And its already mentioned in the kde4 guide: http://www.gentoo.org/proj/en/desktop/kde/kde4-guide.xml#doc_chap5
Seems like the config file was created by kcm-gtk-config. So, I've added a postinst message to that ebuild warning about the need for icons...
