As already mentioned, the dark theme for incognito mode is available for Google Chrome Canary version 91 as a flag. If you use Canary, then you can enable it by typing “chrome: ...

YouTube could be on the verge of gaining a new set of handy features, including a Dark Theme and Incognito mode. The app could also be about to make it slightly easier for users to skip ads on videos.

Another big feature that could make its way to the YouTube app is Incognito mode. The app already lets you pause watch and search history, but that requires enabling both of them in settings.