Yosemite's Dark Mode is a nice way to darken your screen so that it's easier on your eyes, but you have to dig into the System Preferences to enable or disable it. Redditor abspam3 figured out a way to do that with a keyboard shortcut.
All you need to do is open up Terminal and type this in:
sudo defaults write /Library/Preferences/.GlobalPreferences.plist _HIEnableThemeSwitchHotKey -bool true
Then, log out and back in again. Once you're logged back in, you can enable Dark Mode by tapping CTRL+OPT+CMD+T.