Some applications use the selected theme, some don't

[Luca Olivetti]

Hello,

since I upgraded kubuntu to 18.10 one thing that annoys me is that a 
couple of applications (specifically gimp and krita) use a dark theme, 
regardless of the theme I selected (breeze).
I selected breeze everywhere I could: as desktop theme, colour scheme, 
application style and gnome applications style (I'm not sure about those 
names, I'm using a localized version of kde).
I could understand if it happened only for gtk applications, but firefox 
and thunderbird show correctly, and krita is a kde application.
Bug or operator error?
If the latter, how do I correct it?

Bye
-- 
LUca