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-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.