Enable Hidden Secret Dark Theme Mode
Windows 10 comes with a hidden secret Dark theme mode which can be unlocked and tested in Modern (metro) apps:
1. In Registry Editor, go to following key:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize
2. In right-side pane, create a new DWORD AppsUseLightTheme and leave its value to 0
3. That's it. Now you can enjoy dark UI in all modern apps including Settings app.
To restore default UI, delete the new DWORD AppsUseLightTheme.
PS: In previous testing builds of Windows 10, the DWORD was actually SystemUsesLightTheme which was replaced with AppsUseLightTheme in final version of Windows 10.
UPDATE: Now this dark theme feature is officially available in Settings app:
Enable Dark Mode for Windows 10 Settings and Apps
To enable the new dark theme, head to Settings > Personalization > Colors. Scroll down and select “Dark” under “Choose your app mode”.
The Settings application itself will immediately turn dark, as will many other “Universal Windows Platform” applications. However, it’s up to each developer to support the dark theme, and some applications will continue using their normal colors.
This option also doesn’t affect desktop applications, including Windows 10’s File Explorer, which will remain white.