Comment 5 for bug 1868914

Once installed, you cannot remove vanilla-gnome-default-settings.

(Reading database ... 147624 files and directories currently installed.)
Removing vanilla-gnome-default-settings (18.10.1) ...
update-alternatives: error: no alternatives for gdm3.css
dpkg: error processing package vanilla-gnome-default-settings (--remove):
 installed vanilla-gnome-default-settings package pre-removal script subprocess
returned error exit status 2
dpkg: too many errors, stopping
Errors were encountered while processing:
 vanilla-gnome-default-settings
Processing was halted because there were too many errors.
E: Sub-process /usr/bin/dpkg returned an error code (1)

I noticed that running...

update-alternatives: error: no alternatives for gdm3.css

... to get the standard gdm login screen is broken as well on a clean install.