During the 2014 WWDC keynote, Apple showcased a new feature for OS X Yosemite called Dark Mode, although for some reason the first beta never had the option to enable this feature. And, Mac OS X 10.10 update 1.0 doesn’t either. Fear not because there is a way to get this feature to work, you just need to know how.
One Mac developer has managed to work out how to enable Dark Mode within Yosemite, but requires you to use a Terminal command in order to do so. Some of you might be worried about doing such a thing, but if you want a taste of what Dark Mode looks like for yourself, then you will just have to take the chance.
The Terminal command that will enable Dark Mode is /Library/Preferences/.GlobalPreferences AppleInterfaceTheme Dark, and so just copy and paste this in and you are ready to go. After doing this you will then need to log out of your account, and once you have logged back in you will notice that the Dock and Menu Bar are now much darker, along with other menus as well.
9to5 Mac says that because this is an early version of the new mode some of the icons might not show up like they should do, and as always you need to be cautious when enabling such features that are not built into OS X 10.10 just yet.
If you do not like Dark Mode, then you can revert back to normal by removing the key defaults delete or just change Dark to Light.
Also See: iOS 9 OTA beta 3 update notes live