I use the Breeze Dark theme on Kubuntu, which also works perfectly with GTK+2 applications, but it's not fully supported on GTK+3 applications like Firefox yet.
Because of that, I've set my system to use Breeze Dark for GTK+2 programs, and the light Breeze for GTK+3 programs.
However, the KDE settings window (GNOME Application Style) only lets me pick a single icon pack for both versions. Is it possible to use different icon sets for them? The "fallback theme" option doesn't work for that.