I really like the wine, a complete .exe emulator. But I would like to use an Mac OS X theme, so, I would like to use the "Made for Mac" programs.
Where can I download this emulator - if it exists?
Yes there is a compatibility layer that can run Macintosh binaries On Linux and it is called Darling. You can get it by this command in the terminal (Note this is only for the source code)
and you need you compile it also, where is link telling you how http://darling.dolezel.info/en/Build.
Note this project is early in development, there aren't many Apple programs that can run on this compatibility layer. There is more Apple programs that can run on Darwin OS than this compatibility layer that said I think you will be better off running Darwin in a VM!.
I think there is no "emulator", as there are for example nes-emulators. Also, Wine isn't really an emulator.
So, I think the best solution for you would be virtualbox, a program which allows you to install several other OSs in Ubuntu. Install it from the softwarecenter. Open it and put the OSX-DVD in your mashine (This is the only non-free part of this solution). Create -New- and follow the wizard. Your operating system will be installed and you can use it inside Ubuntu, as if it was on a separate computer. Everything inside a window, a little Mac.