
OS X Yosemite's dark mode, which was demoed on stage at Apple's Worldwide Developers Conference, has yet to make it into the beta as an available setting. It is, however, possible to get a glimpse of dark mode with a Terminal command, as discovered by iOS developer Hamza Sood.
To enable the somewhat functional dark mode: sudo defaults write /Library/Preferences/.GlobalPreferences AppleInterfaceTheme Dark - Hamza Sood (@hamzasood) June 17, 2014

Article Link: OS X Yosemite's 'Dark Mode' Now Available via Terminal Command