https://www.techradar.com/news/microsoft-embraces-the-dark-side-with-new-windows-10-file-explorer-theme

Perhaps attempting to keep up with Apple's Dark Mode in the upcoming macOS 10.14 Mojave, Microsoft has announced that the next major release of Windows 10 will bet getting a Dark Theme for File Explorer. 

Available with Windows 10 Insider Preview Build 17733, members of the Windows Insiders program will be able to apply the new Dark Theme by going to Settings > Personalization > Colors and then selecting the 'Dark' option found in the 'Choose your app mode' section.