Back in August 2016, Microsoft introduced the dark theme to Windows 10 with the Anniversary Update. Since then, most of the company's in-house apps, such as Skype, Edge, and even File Explorer, have gotten the dark treatment. One notable holdout was always OneNote.
But that's changing. Aggiornamenti Lumia, as it so often does, got its hands on an unreleased version of OneNote that has the coveted theme. It's not available to Insiders yet, and Microsoft hasn't even announced it, but one thing for sure is that it's coming.
Interestingly, at this time, the dark theme applies to all area of the app except the note-taking area, where the page remains white. This could change, but it might cause problems for people that inked in black, so Microsoft would have to work out a solution for that.
The report also notes that it syncs with your Windows 10 settings. It's currently unclear if there's also an option to override the system settings.
Naturally, this feature is still early in development, so it's unclear when this will make its way to Insiders, let alone the general public. It's also too early to be sure about what the final product will look like.