Log-in screen is wrongly themed in vanilla-gnome-desktop mode

For several releases now I have been running Ubuntu with the 'vanilla-gnome-desktop' package installed. This changes the start-up, log-in and shutdown screens to dark grey with Ubuntu GNOME logos. Highlighting, underlining and sliders are blue on the log-in screen.

Since the update to gnome-shell 3.36 I am seeing the log-in screen as being a purple colour. The log-in box has an orange outline and the indicators in the top bar have an orange underline when clicked. The volume and brightness sliders are also the wrong colour. The colour of the indicator underline and sliders display in blue once logged in.

The start-up and shutdown screens display correctly which makes the log-in screen look very out of place.

Looking at my Ubuntu 18.04 installation the following files seem to be missing:
/etc/alternatives/gdm3.css
/usr/share/gnome-shell/theme/gdm3.css

So presumably the log-in screen is falling back to the standard Ubuntu theme as it can't find the GDM theme.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gnome-shell 3.36.0-2ubuntu1
ProcVersionSignature: Ubuntu 5.4.0-18.22-generic 5.4.24
Uname: Linux 5.4.0-18-generic x86_64
ApportVersion: 2.20.11-0ubuntu21
Architecture: amd64
CurrentDesktop: GNOME
Date: Thu Mar 26 21:04:48 2020
DisplayManager: gdm3
InstallationDate: Installed on 2019-05-17 (314 days ago)
InstallationMedia: Ubuntu 19.10 "Eoan Ermine" - Alpha amd64 (20190517)
RelatedPackageVersions: mutter-common 3.36.0-2ubuntu1
SourcePackage: gnome-shell
UpgradeStatus: Upgraded to focal on 2019-11-08 (138 days ago)