Found the error in the code which causes backgrounds not changing on schemes that has the default black/gray background.

Will upload a fix later today. In the meantime if you don't want to wait, type these into xterm:
sudo gainroot
mkdir /usr/share/themes/blackplastictheme-themedir/defaults/backgrounds
mv /usr/share/themes/blackplastictheme-themedir/defaults/*.png /usr/share/themes/blackplastictheme-themedir/defaults/backgrounds/

After that backgrounds are working again Sorry, my bad and thanks for testing!