They are in the skins folder, C:\Program Files\Alwil Software\Avast4\DATA\Skin, you will probably have to disable the avast self-defence module first, Program Settings, Troubleshooting. The skins don't take up much room though.
The skin file type is .asws so you can remove the ones you added, don't remove the three default ones, silver panel.asws, low res.asws, teak zepplin.asws (I can't remember if the szc-kde.asws is also a default). If you happen to remove the defaults I think they will be restored, though I can't recall 100%.
Enable the self-defence module when you have done.