There are no skins for the QuickTime player itself -- the ability to "skin" in QuickTime involves media distribution, not the player. In other words, the "skin" is carried along with the media itself (be it MPEG4, MPEG2, MOV, etc.), so when the movie is played, the custom interface is used.
QuickTime Media Skins are delivered to users along with media files, allowing digital media authors to create compelling experiences specifically for your own content.
It's for, say, if a company wanted to distribute a new training video in QuickTime format, they could also create a custom interface with their logo or something.