https://wccftech.com/enable-windows-10-file-explorer-dark-mode/

Dark themes have been a highlight of this year with every operating system, browser and app trying to offer a dark mode to its users. While Windows 10 already supported a dark theme, the latest Windows 10 October 2018 Update finally brought the Dark Mode to File Explorer.

The mode was already available to Settings, Photos and other Universal Windows Platform applications. However, File Explorer continued to open with the light theme, which kind of killed the purpose of a dark theme. If you have installed the latest Windows 10 version 1809 (users are advised to wait a few more weeks to avoid any compatibility issues), you can have a system wide dark mode experience by following just a few simple steps.