A while back, we heard that dark mode was finally making its way to the OneNote app for Windows 10, though Microsoft hadn't officially announced it. Now, the company is rolling out the feature to select Insiders, as spotted by Reddit user Grindiot (via Windows Central).
Dark mode does pretty much what you'd expect, turning the entire interface, including the notebook pages themselves, into a dark grey color instead of white, and the title bar also turns black instead of using OneNote's signature purple. Other colors are also more muted compared to the light theme variants.
You may want to be careful, though. Users on Reddit have reported that the new theme also changes the default color for your pens, making them darker. This may make your notes look different than you originally intended them to. It seems like it's possible to change the colors back on an individual basis, but that may take some time if you have a lot of notes.
For now, the dark theme in OneNote seems to only be available for a few select Insiders. On our test machines, the feature isn't yet available, despite being on different rings of the Insider program. Broader availability should come over the next few days and weeks.