szc
If you downloaded them with Firefox as you said yourself, whatever you did, you had to install it because all you've got by downloading it with Firefox is installation file .aswcs and nothing else. You have to install it to extract the skin file from it... Trust me, I know exactly what I'm talkig about, I made bunch of those skins and I know how they work.
I believe you, I just don't remember... (pretty sad, as it was only a week ago)
Ok, I just downloaded a skin...
I put it into the Skin folder, and double clicked it, which produced a larger file with the same name, and a text file, Avast took care of the rest... as for technicalities in case that does not work, I have no idea...