To enable the somewhat functional dark mode: sudo defaults write /Library/Preferences/.GlobalPreferences AppleInterfaceTheme Dark
To undo, either remove the key (defaults delete) or change it to “Light”
Take a look at a screenshot of the result below. We'll be posting on the new features in OS X 10.10 Yosemite soon. Follow iClarified on Twitter, Facebook, Google+, or RSS for updates.
Read More [via Bruce]