There is little benefit in doing this as when avast version 5.0 comes in there will be no skins. This is because the third party skinning software will no longer be available.
There are many other skins you can choose to use with avast 4.8, open the Simple User Interface (where you do on-demand scans), right click on the middle of the skin, from the drop down menu, Select Skin, Get more skins link, see images.