By modifying a simple registry key, users can now add a dark theme to the Settings app and other parts like on the Network and volume control in the Notification area. The new theme works independently from the dark theme that users can enable in Windows 10 build 10074 with the Personalization options in the Settings app.
Enable dark hidden dark theme
If you want to know what else is new, or if you haven’t install build 10074, check out this previous write-up.
Update, May 12, 2016: Starting with the Windows 10 Anniversary Update, you can simply go to Settings > Personalization > Colors and set new dark theme without having to modify the registry. Source WindowsCentral All content on this site is provided with no warranties, express or implied. Use any information at your own risk. Always backup of your device and files before making any changes. Privacy policy info.