The Winamp (classic) skinning was so brutally effective, considering how simple they were. AFAIK the bulk of the skin consisted of two or three .bmp-files which were spritesheet for all controls and fonts. Literally anyone could make one with a tiny bit of effort.

Now that screen resolutions vary quite a bit I suppose it wouldn't work as well as when all monitors were mediocre 800x600 or glorious high res 1024x768. Ah, the nostalgia.