it is the indicator that PC is in AC supply, It wasn't there till cairo dock 3.0 but that comes after 3.1, I am just changing png images and behavior of dock and few images here and there,,,your dock looks cool,,

Hi What did you do to make your modifications? I did some modifications on my dock , but i was not able to fix minors problems, for exemple a blank square before battery icon render. Here is my theme: http://gnome-look.org/content/show.php?content=156653