In Plasma 5, all non-KDE apps only have GTK+ decoration

Regardless of which window decoration I choose, all the non-KDE programs (Firefox, GIMP, Shutter etc.) have the hideous GTK+ style. I’ve already installed gtk3-theme-oxygen, but that won’t help. Maybe I missed some other packages?

You are talking about the widget style here I suppose. The window decoration (window titlebar and its buttons) should be the same for all windows, as it is drawn by the window manager (unless client-side decorations are used, which is preferred by GNOME these days…).

First, gtk3-theme-oxygen doesn’t work at all in Leap 42.1 (or Tumbleweed), because GTK3 has dropped support for theming engines completely in recent versions.
Second, you have to configure the GTK style separately in “Configure Desktop”->“Application Appearance”->GTK. GTK doesn’t follow KDE’s settings.