This happens with the calculator and other default apps as well. Default apps are installed in 18.04 through "snap", a new cross-OS package manager. Snap will be much better than APT one day, but right now it's in the beginning stages and experiencing growing pains.
One of these growing pains is that it "themes" each application differently than it has in the past, and some of the new "theming" hasn't had the kinks worked out yet.
Luckily, the workaround is very very easy. Just uninstall with snap and reinstall with APT.
sudo snap remove gnome-system-monitor
sudo apt install gnome-system-monitor
You might want to do this with gnome-calculator, too. I'm still looking for other applications that have this issue.