Gwibber, of course!
To install, put this in ~/.local/share/gwibber/ui/themes/ and pick the theme from Gwibber's preferences.
GPL
Garrett LeSage
Jakub Steiner
Garrett LeSage
You can download this project in either zip or tar formats.
You can also clone the project with Git by running:
$ git clone git://github.com/garrett/gwibber-gilouche