It was with the Windows 10 Anniversary Update that Microsoft decided to add a dark theme to Windows 10, as it had been doing on Windows Phone for years. Last fall, Xbox users got a light theme option, as the ability to choose between color schemes has become more popular.
Now, the dark theme option is being extended to the File Explorer, as first discovered by Thurrott's Rafael Rivera. The feature is hidden in Windows 10 build 17650 - which is only available to Insiders on the Skip Ahead option of the Fast ring - and as you can see from the image above, it seems to be early on in development. Once it's available to everyone, it can be toggled on and off using the system-wide theme setting.
Historically, the system-wide theme setting has controlled various elements of the general Windows 10 UI, as well as various UWP apps. This might be the first time that the setting has touched a Win32 app like File Explorer. It's good news though, as dark themes have been a fan favorite.
Of course, the feature is unannounced, so it's possible that it could be scrapped at some point, or that it could be delayed until a later update. If all goes well though, File Explorer will get its dark theme when Redstone 5 is available this fall.