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-.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: