First, download from here https://skins.webamp.org/
An article I found through search says the following:
- Windows version needs some massaging to use them. Download the skin, then rename the file so the extension is .ZIP instead of .WSZ. Unzip the file and drag the folder to C:UsersUSERNAMEAppDataLocalaudaciousSkins, replacing “USERNAME” with your Windows username. The skin will show up in the Audacious settings, though you may need to restart the application first.
Good luck.