Before the Anniversary Update release you had to use the registry trick to enable the dark theme for apps in Windows 10. Now you can enable the dark theme in one click using the option in Settings.
How to enable the dark theme for apps in Windows 10 Anniversary Update
Step 1. Press Win+I to open Settings.
Step 2. Go to Personalization → Colors.
Step 3. Scroll down until you see the setting Choose your app mode and switch the selector to Dark.
The settings will be applied immediately: