To activate dark mode in OS X Yosemite you have to take many steps, Apple doesn’t made it possible to activate dark mode with a hotkey. You can activate itself with the following command. Launch Terminal and enter the following command.
sudo defaults write /Library/Preferences/.GlobalPreferences.plist _HIEnableThemeSwitchHotKey -bool true
You may be asked for your administrator password once you hit the return-button. Restart you’re Mac and now you can toggle the Dark Mode on or off using the CMD+ALT+CTRL+T hotkey combination.