does not apply to GTK3 apps, which most GTK apps in recent distro versions are. I suggest to investigate use of ~/.config/gtk-3.0/gtk.css and ~/.config/gtk-3.0/settings.ini to produce your desirable customizations. My gtk.css includes the following:
Users and admins shouldn’t be touching /usr/share/ content with text editors. /etc/gtk-3.0/ and ~/.config/gtk-3.0/ are appropriate places. Conflicts between them and /usr/share/ are resolved against /usr/share/, while valid content in them that isn’t addressed in /usr/share/ is nevertheless supposed to be applied.
this worked for me for browser and interface. But not for example if I open a .pdf with a reader like okular.
Do you know how to get the same for all applications?
I don’t know, but I’ve not seen any exceptions other than LibreOffice, or many moons ago when last tried, WINE. Okular I don’t use. I use Kpdf or SeaMonkey.