The skin I saw most often back in the day had some dumb non-rectangular shape with a "texture" on it and the controls scattered all over the place. Right in line with the "skinning" fad of the early 2000s, which defeated the intuitiveness of GUIs and decades of their evolution.

Then came the even-dumber "transparent-UI" fad.

Fortunately both those died. Oh but wait: Here comes Apple, exhuming one of them in 2025! But Apple has always hated customization, so they probably won't be resurrecting "skins" too.