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
The second beta of OS X Yosemite was released to developers earlier today, bringing several new changes like the return of Photo Booth and a new look for Time Machine. The public release of the software will likely come in the fall, after several more beta iterations.