Although the dark mode for OS X Yosemite that Apple previewed onstage at WWDC two weeks ago hasn’t officially made it into any build of Yosemite as of today, developer Hamza Sood has figured out a Terminal command which will let you get a taste of OS X’s dark mode now:
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Just paste the line below into Terminal, and log out of your account. After logging back in, you’ll see that the Dock, Menu Bar, and corresponding menus all take on the new dark theme.
Of course, this is an extremely early version of dark mode, and many of the icons in the menu bar appear to be displaying incorrectly. As always, use your own discretion when enabling features like this. If you’d like to revert back to the default OS X style, there’s a Terminal entry for that too: