Some old and mature GTK2 apps are still used by users, and they are not maintained, so the probability of them being ported to GTK3 or 4 is near zero. And since Plasma desktop deprecated the support of GTK2, normally it should not interfere with any GTK2 config files, but the reality is that upon any change of Plasma desktop theme the content of the file "~/.config/gtkrc-2.0" is reset which force all GTK2 apps to always use Light theme even if the user already configured them manually to use dark theme.
When you say "Plasma desktop theme" do you mean "Global theme"? Because to my knowledge there is no code capable of changing GTK stuff when you change the Plasma theme.
(In reply to Nate Graham from comment #1) > When you say "Plasma desktop theme" do you mean "Global theme"? Because to > my knowledge there is no code capable of changing GTK stuff when you change > the Plasma theme. It happens when I change the global theme like shown in the attached video.
Created attachment 158632 [details] Changing desktop theme resets gtkrc2.0 file content