https://www.bleepingcomputer.com/news/microsoft/windows-10-insider-build-17733-released-with-finished-file-explorer-dark-theme/

Microsoft just released Windows 10 Insider Preview Build 17733 to Windows Insiders in the Fast Ring. The main improvement in this build, is the finalized version of the Dark Theme for File Explorer. Otherwise, it is mostly bug fixes and minor improvements as Microsoft continues to finalize the development of the Redstone 5 feature update.

Back in May we reported on a dark theme being added to File Explorer in Windows 10 Insider Preview Build and it has gradually been worked on in subsequent insider builds. With this release, the dark theme is finally finished in File Explorer and will be available to everyone when Redstone 5 is released around October.