Well, the upcoming macOS Mojave comes with DarkMode appearance. Do we need to install 10.14 just to "feel in the dark"? Definitely, No.
So, with these few steps we are able to get it (on my case, I'm running macOS 10.13.5):
1. Open Utilities - Terminal.app
2. Run this command:
defaults write -g NSWindowDarkChocolate -bool TRUE
3. Log out via Apple menu, and then log back in again.
4. For full DarkMode, you may consider to check: Preferences > General > Use dark menu bar and Dock
Now, enjoy Mojave-ish looks a like macOS High Sierra with DarkMode.
Feeling not so eye catching with this hack? Just restore the appearance using:
defaults delete -g NSWindowDarkChocolate
Original credit goes to Steve Troughton-Smith: