Ok... what happened was, you installed the Tiger Sierra Theme within Tiger, then upgraded to Leopard without uninstalling the theme. I can tell because of the "macOS Tiger" image and the Apple logo, which was my work. Since Leopard Rebirth (and the Leopard upgrade install, for that matter) doesn't modify the SecurityAgent in Leopard, all of the files that Tiger Sierra replaced are still there, and Leopard WILL use them if they're present. I know this because I have my custom black outline buttons (the "Back" and "Log In" buttons shown in your picture) installed on my Leopard machine.
Fortunately, I have a fix. Download the script below, unzip it to your desktop, then open a Terminal window. CLOSE ALL OTHER APPLICATIONS then follow these instructions:
- Type cd and a space. Drag the SecurityAgentFix folder to the Terminal window and press return.
- Type "sudo ./securityagentfix.sh" (without the quotes) and press return. Enter your password.
- Let the script do its work. There will be no prompts or output except for when it's doing the permissions repair and is finished removing the files. DON'T STOP THE SCRIPT AT ANY TIME!
- Once it is finished, it'll do a permissions repair. This will take some time.
- Once permissions are repaired, your system will reboot.
This should fix your login window.
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OK, then that looks like an issue with Leopard Rebirth.
Might ask in that thread (somewhere around here) or you could try BootXchanger. It's an image. I just don't know the filepath and how to tell you to switch it (other than using Boot Exchanger).
I think though Leopard Rebirth was for Tiger. Not sure though.
The image is in /System/Library/CoreServices/SecurityAgent.app/Contents/Resources/MacOSX.tif
Removing the image causes Leopard to use whatever its default text/image is, stored in one of the artfiles (I think; it could just be text. SST would know more about this since he modified it to say "macOS").