The "gnome" platform theme that's part of the generic unix theme is never selected by applications.

Here's what happens:
QGenericUnixTheme::themeNames, when it detects a GTK-based DE, puts "gtk2" followed by "gnome" onto the theme list. Let's assume the gtk2 theme is not installed. In that case, QGenericUnixTheme::createUnixTheme later gets called with "gtk2" as parameter. Since this is not a name it knows, it returns the generic unix theme. This breaks icons in all Qt-based applications.

I think what should happen is that QGenericUnixTheme::createUnixTheme returns NULL when called with "gtk2". Next, it will be called with "gnome", it will returns the gnome theme, and proper icons will be used.