https://wccftech.com/windows-10-rs5-dark-theme-file-explorer/

While Skip Ahead waits to test the 2019 Windows 10 Preview Builds, Insiders enrolled in the Fast Ring have been in for quite a treat with back to back Builds coming their way. Microsoft has just started rolling out Windows 10 RS5 Insider Preview Build 17733 to Insiders in the Fast Ring. Microsoft said that the company is finished doing what it set out to do for this release when it comes to user-favorite Dark Theme for File Explorer.

With Build 17666 we started our journey bringing dark theme to File Explorer. Today’s build marks the turning point where we’ve finished what we set out to do for this release. Thank you to all Insiders that have shared feedback during this time!