The .car File Explained
Sometimes, usually inside bundles, you come across .car files. And they look promising.
Unfortunately, the .car file format is not really documented, and it cannot be opened with Xcode although Xcode is able to generate .cars. If you look around the web, there are a few tools for peeking inside .cars, though. One of them is ThemeEngine by Alex Zielenski (download here), another one is Crunch by Pragmatic Code. Haven't tried that one yet, though.
If you intend to do some themeing, i.e. want to manipulate the system's .car files, please make sure at least twice that you have a working backup of those files! (MacConfig)