Yosemite’s new Dark Mode is one of my favorite new features in OS X 10.10, but toggling it on and off can be a pain.
Apple didn’t include a hotkey combo that lets you switch from Light to Dark mode, but with after digging through Yosemite’s code, someone has discovered a single line of code you can paste into Terminal that will allow you to dynamically switch to dark mode without digging through your System Preferences each time.
Here’s how to do it:
1. Open Terminal
2. Copy and Paste the following into the command prompt: sudo defaults write /Library/Preferences/.GlobalPreferences.plist _HIEnableThemeSwitchHotKey -bool true
3. Log out of your Mac and log back in.
4. Hit CTRL+OPT+CMD+T to toggle Dark Mode off and on.
Source: Reddit