Very simple dark transparent plank theme that works well with gtk themes that have orange as their selected color, like Ubuntu Light Themes (Ambiance, Radiance) or my very own ImmUnity.

Extract the .tar.gz to ~/.config/plank (Yes, it will overwrite the previous theme, so make a backup, if necessary.)

To position it to the top left corner directly bellow your top panel (as can be seen on the screenshots) to sort of imitate the Unity Launcher, then edit ~/.config/plank/dock1/settings :
Position=0
Offset=-100