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If you are running the latest Windows 10 Insider Preview build (10074) and would like to try out the new dark mode, here's how you can do just that with a simple registry entry. We highly recommend you backup your system before messing around with the registry. When you are all set, continue reading below to see how you can enable dark mode on Windows 10 build 10074.
■ First, open up Cortana and begin typing "regedit" and she will give you the regedit run command.
■ Go to HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize.
■ Right-click and "create DWORD" using the key SystemUsesLightTheme
■ Set the DWORD value to 0, which it should already be by default.
Enjoy dark mode on Windows 10 Insider Preview build 10074. Clearly there is a lot of work to be done but the dark mode is a nice start and welcomed for those who hate staring at a bright screen. If you get tired of dark mode, don't worry, you can easily change it back by setting the DWORD to 1, rather than 0.
source:winbeta