Set a screensaver as a background for Desktop

Quick Google search revealed the following code:

set tlst to {}
--set newtop to (path to home folder as string) & "Pictures:desktop pics folder:BigDipper 916.jpg" as alias -- sample line
set newtop to choose file


tell application "System Events"
   set tlst to a reference to every desktop
   --set picture of item 1 of tlst to newtop -- display 1
   set picture of item 2 of tlst to newtop -- display 2
end tell

Taken from

http://macscripter.net/viewtopic.php?id=35840

I don't have a second monitor so I cannot test it.

Hope it helps.

According to

'AppleScriptEditor' > File > Open Dictionary >
   Finder > Legacy Suite > Application > desktop picture

all you can do is set the picture.

There are references to messing about with the .plist file, but the internet (and Apple) are silent on an Applescript-approved route.

This plist is empty for me in Sierra: ~/Library/Preferences/com.apple.desktop.plist so I imagine Apple have hidden what used to be there inside one of their even less well documented databases.