The default skin is the skin shown without any changes to the configuration, so it's probably the one you have.
To "see" it, you have to start the user interface of avast. Click on the avast! blue (a) icon in your taskbar with the right mouse button and select Start avast! Antivirus - the second option from the top.
Or, you can select "avast! Antivirus" from the Windows Start Menu.