With a single user on MacOS the login screen background should be a blurred version of the desktop background. However sometimes when picking a new desktop background the login background defaults to standard MacOS background. To resolve this do the following.
- Pick a different desktop background to the actual one you want to use.
- In the Finder, choose Go > Go To Folder.
- Paste the following and press return: /Library/Caches.
- Delete the file named com.apple.desktop.admin.png.
- In the Desktop & Screen Saver preference pane, select the desktop background you want.
- After a few seconds the com.apple.desktop.admin.png should be recreated using your new background image.