I have a few Gnome programs. They don’t use my KDE theme (the default, didn’t change anything there). Especially for opening of saving files that’s annoying (and ugly, because I don’t like the Gnome look 
).
For Zettlr (not Gnome I think, but it also used a different style) I could change that in the application settings: GTK_USE_PORTAL=1 as environment variable.
So I could change that for each Gnome application. But maybe there’s a global way of doing this?