If you run Ubuntu or a different distribution that uses Snap packages, you may have some apps even preinstalled as Snap packages (i did not use the Softwarecenter like at all but had gnome-calculator and gnome-system-monitor as this kind of packages).

There is a known issue with this kind of packages, where the GTK-theme is not applied correctly. In this case you may replace your packages, here is what I did:

snap remove gnome-calculator gnome-system-monitor;
sleep 1;
apt install gnome-system-monitor gnome-calculator

...and everything is fine again.