Downgrading GTK3

After installing GTK3.20, my Ubuntu 16.04 installation is now completely unusable.
The ambiance theme is broken, the password field in lightdm has the wrong coloring and the session switcher completely malfunctions (it crashes before showing even one entry). Furthermore, the option wheel in the upper right coer of lightdm is completely gone.

Is there any way I can downgrade back to GTK3.18? Purging libgtk-3-0 didn't help, as it installed libgtk3.20 again, once I tried re-adding ubuntu-desktop.