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. Rafael Rivera posted some information on Twitter:

(mach2 enable 10397285) pic.twitter.com/PI5JKJAid2 — Rafael Rivera (@WithinRafael) April 20, 2018 Since, the dark theme for the File Explorer is in its very early stages, there is no clarity if the dark mode would ship in File Explorer with Redstone 5, or would it postponed to a later update. Source: Thurrott