Skins, no longer exist, so you are seeing an unchangeable User Interface in regards to a very long time ago (Avast 4.x era) that skins were used. Whilst there were lots of nice skin options then, one of the reasons skins disappeared was the unavailability of the 3rd party software used to run the chosen skin.
Each UI from that point forward was a custom UI, I can't see Avast going backwards (something which they rarely do) and changing the current UI, which I personally dislike, too big in your face, a bit gaudy.
I'm still using avast 12 on one of my systems, my win7 netbook with tiny 1024X600 screen, the UI would just swamp it.