It isn't a skin as such, not in the same way as the old avast 4.X era had skins and the user could switch to any of the default skins or download additional skins. There were some great skins for avast 4.x.
Avast made the decision to have a different colour scheme between the Free and paid versions. I too would have preferred a darker scheme.
There were a lot of avast users complaining about the white UI as contrast was an issue for many, not to mention bland.