Short Bytes: Now activating Dark Mode in YouTube is even simpler. As it’s rolled out officially on an opt-in basis, you may activate the Dark Theme and Material Design with a single click. This move is in line with Google’s move to push a cleaner look across all platforms.
Update: Dark Mode is now available without any tricks:You simply need to visithttps://www.youtube.com/newand hit the Try Now button. After doing so, choose Dark Theme option from the profile icon or hamburger icon if you aren’t signed in.

The old trick continues from here:
This secret Dark Mode in YouTube was uncovered by a Redditorpaul-, who found the menu and shared with the others. If you like staying up late till the morning watching YouTube videos, this YouTube trick can be pretty helpful to you.
Before going ahead and knowing about how to activate YouTube Dark Mode, check out this list of some cool YouTube tricks:

Enabling the YouTube Dark Mode is very simple. Follow the steps told below and activate it.
Note:Make sure that you’re running the latest Google Chrome, i.e, version 57 or higher

Activating YouTube Dark Mode
Here’s how YouTube looks in dark mode:
If you wish to get it working in other browsers like Chrome and Firefox, you may visitthisReddit thread to know the steps.
At the moment, it remains to be seen if YouTube would launch this feature officially in a future update. Meanwhile, use this hack and save your eyes.

Recommended:A Complete Guide On YouTube Tips and Tricks
