Perhaps receiving a kick in the pants from Apple with its new dark theme for macOS, Microsoft has added its own take for Windows 10's File Explorer, as part of a recent Insider build.
Build 17733, released on August 8, features the new dark theme, which can be enabled via Settings -> Personalization -> Colours.
Unsurprisingly, a dark theme has been high on the request list, however, there were a number of technical issues standing in the way of its implementation, as Microsoft's Insiders blog explains:
...compared to modern parts of Windows, File Explorer was a whole different beast. While dark mode can be applied to modern XAML components in Windows quite easily, File Explorer contains legacy UI frameworks which don’t plug into that infrastructure automatically.
With File Explorer, we were literally breaking new ground to provide a dark theme to legacy parts of the shell.
It goes on to say that any changes to File Explorer have to handled delicately, as a lot of applications use the UI for file / folder selection and a misstep could break them.
While the dark theme is only available via Insider, it's only a matter of time before it finds its way into Windows 10 proper. If you've waited this long for it (decades, if we go back to Windows 3.11), I'm sure you can hold out.