Empathy chat view has been supporting Adium themes for a long time now. Those themes are made of HTML, CSS and JS and are rendered using webkit-gtk. Empathy also supports build-in themes using a plain GtkTextView.
I’ve been working recently on improving empathy’s code to support the most recent theme spec as well as fixing rendering issues we had with some themes. I also added a theme preview is settings dialog.
Designers have been working hard to create the GNOME 3 look & feel, and I think a GNOME adium theme would be great to have for empathy chats. That theme would be shipped by empathy and set by default, so we could drop our GtkTextView-based themes for a massive code simplification.
This is a call for designers: We would like mockups, ideas, and finally a working adium theme to be used by empathy by default.
In a first step, gimped image of what we would like would be enough. Then we can start implementing it with the HTML/CSS.
Suggestions and ideas can be posted on the empathy bug #645920, or on the usability mailing list.