It is literally a zip file. If I remember correctly you have to write some xml which describes the layout of the skin and then include any images you need, you then zip it and change the file extension to wsz. So really you could put anything in a skin.
This is a common design pattern for “packages”.
Winamp uses a pattern called package management for its skins. Same as chrome extensions, etc etc. Most of the time we don’t call it that because the term is more reserved for package sets which provide a wider array of functionality. The iOS app store is a package management system.
The packages, much like boxes being handled by FedEx, contain standardized elements which allow that package management system to find and install the package. Much like how when you ship a box through FedEx the first thing they do is put standardized labels on that contain all the different barcodes that different parts of their process use to route the package.
Like with Chrome extensions, there are certain files that must be there (iirc something like a manifest.json file) and others which can be anything. If you need an image for a button, that image becomes part of the package. If you need a complex set of rules then maybe there’s a little sqlite file or csv file containing all the rules. Or font files, or whatever.
Thing is, the package format is defined by the “box” as opposed to by the “contents of the box”.
Just to put this in context:
There’s only so many ways to turn a bunch of files into one - mainly, you stick them back to back. Easy.
Then, there’s an infinite ways to compress that file… You could come up with you own method, but what good is that? It’s better and smarter to use a format already supported by your users
So of course most bundles are the same archive type under the hood. Everything from backups to installers - you shouldn’t be inventing new formats without a damn good reason
