Download and Extract this zip file here.
Copy the contents to C:\Windows\System32\oobe
Copy the info folder "info" here. In the info folder there will be a backgrounds subfolder.
In the backgrounds folder rename the desired background colour to "backgrounddefault". This by default will be light blue.
Alternatively replace the backgrounddefault file with any jpg of your own as long as it is less than 250 KB.
Run the registry file..
This will change the Lock Theme. The following colour schemes are included for the Dell OEM Settings, the default one is light blue.
Windows FAQs and OEM Downloads