Enabling the mode is fairly simple, provided you're using an up-to-date Chrome browser. You'll need to access the browser's developer tools and paste in some text from the Tweet above, refresh your browser, and then toggle the mode on in the menu you get when you click on your avatar. You can easily turn the mode off, too, though you only have to use the console command once.
It's an easy, semi-official way to save you some eye strain. Even though Dark Mode hasn't been announced by YouTube yet, it could be a great regular feature that may not require all the extra steps in the near future. Why not give it a try now?