Microsoft has added a hidden feature that enables a dark theme in File Explorer. The feature is in very early development and requires a special tool to enable it. Windows expert Rafael Rivera discovered the feature in the latest Redstone 5 skip ahead build for Windows 10 Insiders. With only the tab bar and navigation bar currently using the dark background, this theme currently looks awkward at best. For those who want to play with it, you can compile the mach2 application and enable it using the “mach2 enable 10397285″” command.”
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-is-adding-a-dark-theme-to-windows-10s-file-explorer/