Microsoft is definitely listening to proposals and feedback of Insiders. It would seem that the request of Windows 10 dark theme has finally been accepted and will come for everyone with the next update to Windows 10, in July.
Unlike changing the color theme to Windows 10 through the current Settings Menu or going through each app to apply a Dark Theme, the upcoming Windows 10 Anniversary Update Dark Theme will be used current Windows Phone and Windows 10 Mobile devices implement the feature.
Nothing exceptional, but for now, you can officially enable black theme on Windows 10, same as Windows 10 Mobile. According to some rumors, theme would be enabled in some internal builds.