At Ignite earlier this year, Microsoft introduced a revamped OneDrive sync client for macOS, which also included a new dark theme to go along with the release of macOS Mojave. At the time, the company said that the client on Windows wouldn't be receiving support for the dark theme due to it being a Win32 app, but those plans seem to have changed.
Twitter user Florian B shared an image of a version of the sync client which does include a dark theme in the latest Windows Insider build. As per On MSFT, you don't however have to be on build 18277, released earlier this week, for it to show up.
The dark theme comes with version 18.212.1021.0007 of the OneDrive sync client, and that update seems to be still rolling out. As of the time of writing, Microsoft's dedicated support page makes no mention of it, and our test devices haven't received this update yet, so it may take some time for it to show up for you as well.
The arrival of the OneDrive dark theme should add some much-needed consistency to Windows 10's look overall, but it's interesting to see how quickly Microsoft changed its plans in this case. The update should make its way to general users in the future, but it remains unclear when that will be.