Any good windows system APIs?

I wanted to know if you have seen any window system API's that support "skins". I don't want the game I'm making to have the "Windows Default". I tried using CEGUI. (website here)

I'm a n00b when it comes to window systems. I have only used a couple APIs before...:/ If you can get me going in the right direction that would help a lot. Thanks.

To me those would appear to be GUI subsystems (quoting from the link) in which case it would only really be platform specific if it was a platform specific graphics library that it was coded in.

I don't know what the specific name for it is. It seems like GUI is a little broad.