I personally appreciate this move since the darker theme will make the viewing experience less stressful on the eyes when you're watching content for hours. Now you should be able to select a "Dark Mode" from the settings list.
Windows users can do this by pressing Ctrl + Shift + I.
That said, you're welcome to enable "dark mode" any time you want, if you prefer the contrast provided by its color scheme.
Williams shared his excitement about a hidden dark mode on his Twitter account, sharing a browser screenshot that shows YouTube with a fully functional dark mode running.
Paste in the following command: document.cookie="VISITOR_INFO1_LIVE=fPQ4jCL6EiE" into the console window and press enter. Those signed in can click their profile icon and scroll down to "Dark Mode".
"Refresh the page, if the new layout is enabled, go to your avatar, and from the menu there should be a dark mode toggle".
As it turns out, there is a way to turn on dark mode, as discovered by Owen Williams.