Comment 3 for bug 1756187

After playing around with gconf, dconf, xfconf for a long time I ended up getting my 16.04 system to reproduce this problem, which is not really what I expected at all. I'm still not 100% sure what is going on but I think I am comfortable with recommending to set the /Net/FallbackIconTheme key by default to "gnome", for the following reason.

I found this commit: https://github.com/GNOME/gtk/commit/a093cd2a22173369424878eb6d8c38124e7aa1f9

This is the commit I mentioned above which hard codes a (undocumented) check to "gnome" icon theme in Gtk+3. Note what the comment says:

  "This is what we used to get through the Net/FallbackIcontheme
   setting. Nobody has ever set this setting to a different value,
   and people have come to rely on GTK+ applications getting their
   icons this way."

Well, currently, Xubuntu does set this differently - by not setting it at all.

I have looked everywhere and this seems to be the only setting that controls the fallback icon theme in Gtk+2. Note that Gtk+3 apps don't just have this setting by default. They are hard coded to check "gnome" regardless of the value of this setting. So by setting it we would make our Gtk+2 apps behave a tiny bit more like Gtk+3 apps. (And I won't have broken icons in my menus. :)