Clear cached frameworks to reload OSX UI (graphics only)
If you're looking to re-label parts of OS X, the file you would want to modify is Localizable.strings. Within each language, or the current language of your OS, for instance, en.lproj/Localizable.strings for English, modify the necessary strings. It is necessary to only restart the app for it to take effect.

If you're looking to change the theme of OS X, there used to be various apps that could do it. For Yosemite, the only one I know of is Flavours 2. It includes various themes, and you can also make your own.



http://flavours.interacto.net

For menu bar icons, there are a few menu bar icon packs, like the following;

http://aaronolive.deviantart.com/art/Menu-bar-Icons-OS-X-Mavericks-10-9-x-406393600

though I'm not aware of any apps that work with Yosemite that allow you to change them automatically. You would need to find the correct folder to place the icons, like Yourapp.app/Resources/.