Gtk app themes under KDE

Knowing that we have some theming issues, I recently installed KDE on one of my systems to check the story there. This is the gnome-calculator snap with the latest stable gtk-common-themes. KDE is unmodified, so it defaults to Breeze theme for both Qt and Gtk apps. AFAIK, KDE uses both XSettings and gtk-[234].0 ini files to set the theme and the font.

This is wha native app looks like:

And this is gnome-calculator from the snap:


I am fairly certain that Plasma does not use xsettings, only the ini files.

Defaults are written by this program on (first) login:

That said, besides actually reading the settings the gtk-common-themes would also need to include breeze-gtk and breeze-icons to theme properly.