No, the avastUI is fixed as such and reflects the avast corporate colour scheme in the avast web site, blog and forums, etc.
The self-defence module is also there to prevent malware from modifying/stopping avast.
I have been using avast for over seven years and the avast5 & avast6 GUI is much better, than avast 4.x even with skins it wasn't very easy to navigate and consequently users didn't find it that friendly. Not to mention in reviews the most commonly talked about thing was the quirky media player interface, not how good the features, protection, etc. were.