I have two monitors in a Lubuntu setup, and am only able to change background color on the main monitor, while the other keeps the default Lubuntu wallpaper.
Now, on some other setups, the problem is that they don't use "superwallpapers" that covers the whole area of the combined resoutions of the monitors, as this suggests. But I mere want my "super"desktop area to be black, which I choose under Desktop Preferences → Wallpapermode: Fill with background color only". So I don't understand why it doesn't just fill up the "superdesktop" with black.
Myt setup is an HMDI connected monitor as main monitor in portraint mode (1280 x 1024) and an DVI connected monitor to the right in standard landscape mode (1900 x 1200), as pictured here (not high enough rep to post it directly).
I used Arandr to set it up like this:
#!/bin/sh xrandr --output DisplayPort-0 --off --output DVI-1 --mode 1920x1200 --pos 1024x80 --rotate normal --output DVI-0 --off --output HDMI-0 --mode 1280x1024 --pos 0x0 --rotate left
Hope you have a clue for this. Thanks!