This is a custom UI I built to showcase the collection we started on the Internet Archive as part of that collaboration. The collection has grown considerably since that time, but it’s the same set of files.
What makes Winamp so themeable in comparison with today's softwares? Why we can't do the same today? Take windows itself, it use to have theme packages, but these days max you get is a different color scheme or a custom background only (chrome, Firefox etc ) .
User defined themes place a meaningful restriction on how the UI can evolve. My theory is that as software has moved to the web, and thus a more iterative/continuous deployment model, this restriction has felt relatively more expensive to commit to.