Ubuntu – How to change the theme of Snap Apps

If you open the "System Settings" app, there's an icon in the top row for something like "application settings" (I think it's second or third from the left in the top row--I don't have KDE available at the moment). If you click on that icon you'll see a dialogue with a few options in the left-hand panel. One of them is to set the theme for GTK applications. You should just be able to select qtcurve from the drop-down menu in the right-hand pane under that tab.

It seems those are applications installed as snap packages. You may verify it by running

snap list

The snap execution environment doesn't see any of the themes on your system (local or system-wide). So this is somewhat an expected behaviour.

As a workaround you may remove those snap applications by running

snap remove <package-name>

and install their apt versions by running

sudo apt install <package-name>