I recently installed Xubuntu/Xfce session using Ubuntu Software Center in order to test how Xfce would look like.

I didn't like it, so I removed those packages and got back to the default Unity shell. But now the notifications that appear in the upper right corner no longer look like they should. Instead of that default transparent black design now there is a grey window, like this:

(This is an example from Pidgin, I don't know if it has some importance).

Everything looks ok but those notifications. What can I do to have back the original style?