Microsoft seems to be working on the possibility to darken the appearance of Explorer in Windows 10. The function now only makes the bar on the left side dark and must be activated with a hack.
At the moment the function only makes the sidebar with shortcuts darker and the main window and the menu bars white. It is not clear whether Microsoft is actually planning to further develop the feature and make it available to all users.
The option for a dark theme would not be the only feature for Explorer that Microsoft is working on and demanding . The company is also currently working on the possibility to add tabs to Explorer and other programs. That function should be available later this year with an update.