I have the following in my ~/.gtkrc-2.0:

gtk-icon-theme-name="gnome"
gtk-fallback-icon-theme="breeze"

Still, when I do the following in a test app:

        qDebug() << QGuiApplicationPrivate::platformTheme()->themeHint(QPlatformTheme::SystemIconThemeName);
        qDebug() << QGuiApplicationPrivate::platformTheme()->themeHint(QPlatformTheme::SystemIconFallbackThemeName);

The output is "gnome" twice. It should be "breeze" the second time.

As a result of this bug, many icons are missing in Dolphin and other KDE/Qt applications when running under GTK-based DEs because there is no fallback to breeze.