Change all backgrounds to either light or dark, depending.

chbg.sh

case $1 in
	light)
		imageFile="/Library/Desktop Pictures/Solid Colors/Solid White.png"
		;;
	dark)
		imageFile="/Library/Desktop Pictures/Solid Colors/Solid Gray Pro Ultra Dark.png"
		;;
	*)
		imageFile=""
		;;
esac

if test -f "$imageFile"; then
	osascript << EOF
		tell application "System Events"
			set N to count of desktops
			repeat with i from 1 to N
				tell desktop i to set picture to POSIX file "$imageFile"
			end repeat
		end tell
EOF
		
fi