A command entered in the OS X Yosemite Terminal allows users who are in beta to activate the 'dark mode' or as they call it, the Dark Mode to the operating system. This is something that they did not say in the last WWDC of June, but that allows the user change the look of the new OS X with darker tones.
The new Dark Mode in OS X Yosemite we do not know if it is available in its final version, but it would be great if they left it permanently for those users who want to see the Dock and the upper menu bar with a different look. This change in design has been discovered by developer Hamza Sood and if you are one of those you created a partition to have the operating system installed on your Mac, now you can test it by entering the following command. 
To activate this mode we have to have the latest beta released by Apple and open Terminal by copying this command line:
sudo defaults write /Library/Preferences/.GlobalPreferences AppleInterfaceTheme Dark
Once the phrase is copied, we press enter and then we have to close the session or restart the Mac for this effect of Dark to take effect.
If what we want is to return to OS X Yosemite with light tones that it brings from origin, all we have to do is access the Terminal again and change the word Dark to Light at the end of the command line and that's it. The problem with the black version is that the text on the right side of the menu bar is black and you cannot see anything because everything is the same color.
This ability to change the look of OS X gives it an overall elegant touch, but It is not something that we can use well due to the theme of the menu bar. On the other hand, it is good that Apple allows changing the original appearance of OS X in such a simple way, and I do not remember any previous OS X that allows this modification from the Terminal.