Last month, the Windows 10 Twitter app offered up a dark mode, and now the whole OS is doing it. If you're a Windows Insider, you can grab the new 14316 build and try it out for yourself now. Note that some UWP apps won't be affected by dark mode.
That's far from all: also included is notifications customization (choose priority level and amount of notifications per app), native Bash support (for you Linux lovers), more cross-device Cortana syncing, a 'Find my phone' feature (locate your phone with your PC), a slew of bug fixes, and much more.
If you want the nitty gritty, hit the source.
