Recently I installed Ubuntu 17.10 which comes with Gnome as the default DE and I like it a lot but cannot figure out how to change the purple background of the lock screen (when typing in the password).

I've tried reconfiguring GDM but it seems to be different and it only affects the login screen.

If there is no solution I'm interested in ANY workaround because this purple drives me crazy but I don't want to drop GNOME entirely just because of this. Any help is highly appreciated.