pretty good addon. also works reliably with use of containers. even if you switch back to light theme in youtube settings, this addon changes it to dark theme again.
only drawback is, that on my setup the youtube is reloading twice, and youtube settings list is flickering till theme is finally set.
(youtube itself is dumbly made in regards of theme switching, else it wouldn't need to reload the whole website)

Does what it says. Forces Youtube's Dark Mode on. This setting reverts on Youtube constantly. Sad an extension needed to be made for this.