Lets say a skin app is designed to change a Windows square based UI, then Microsoft launches a new OS that will no longer have those squares, instead it will only have triangles, the skin appeal no longer work. You’ll have to change it to start working with triangles now.
The Windows UI used to use square based UI and people loved it.
Windows switched to triangles, and instead of people being forced to become well versed and proficient with the triangles, the skin apps said Poo-poo.. we will make the triangles look like squares again so nobody ever becomes proficient with triangles or ever accepts triangles.
Then, when the triangles change shape a little bit, to.. trapezoids, we will still have to use squares to customize!!!
No matter the analogy, its a bucket load of crap until the trapezoids and triangles are skinned.