https://www.techradar.com/news/windows-10s-latest-preview-converts-file-explorer-fully-to-the-dark-side

Its focus may be shifting to the next big update for Windows 10 due in 2019, but Microsoft is still continuing work on the build due to land later this year, and has just unleashed the latest Redstone 5 preview, declaring that it has finished implementing the dark theme for File Explorer.

The dark theme was first introduced to Windows 10 back in May with build 17666, and now with the latest build 17733, Microsoft has announced that work is finally finished on the mode, and it’s now a complete experience. In other words, what testers are playing with is what the final dark theme will look like when the big update is released later this year.