Note that GDM is not graphical and has no theme. More likely you're thinking of gnome-shell which also draws the lock screen. The theme for that is provided by yaru-theme.
** Package changed: gdm3 (Ubuntu) => gnome-shell (Ubuntu) ** Changed in: gnome-shell (Ubuntu) Status: Incomplete => Fix Released -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to gnome-shell in Ubuntu. https://bugs.launchpad.net/bugs/1865996 Title: GDM Theme Inconsistant Init Login vs Screen Lock Status in gnome-shell package in Ubuntu: Fix Released Bug description: The initial login theme of first login at machine startup is completely different (looks much older) than the theme used for when the machine is restored from screen lock (looks very new). Ideally the same theme is used in both circumstances for consistency. ProblemType: Bug DistroRelease: Ubuntu 20.04 Package: gdm3 3.34.1-1ubuntu1 ProcVersionSignature: Ubuntu 5.4.0-14.17-generic 5.4.18 Uname: Linux 5.4.0-14-generic x86_64 ApportVersion: 2.20.11-0ubuntu18 Architecture: amd64 CurrentDesktop: ubuntu:GNOME Date: Wed Mar 4 08:14:55 2020 InstallationDate: Installed on 2020-02-12 (20 days ago) InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Alpha amd64 (20200124) SourcePackage: gdm3 UpgradeStatus: No upgrade log present (probably fresh install) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1865996/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp