I followed the link you gave. For the Windows Desktop App, it says to click on the gear next to your username in the left navigation panel. In the current iteration of Evernote, my username doesn't even appear in the left navigation panel. Also, I went through all of the options and, like Shackleford91 said, the only mention of dark mode is dark mode for the left navigation panel. That leaves a whole lot of very bright white on the screen.