Selecting the new Windows Light theme will also change your PC to be light themed. As part of this work Microsoft is also adding a new default wallpaper. The system UI includes the taskbar, Start menu, Action Center, touch keyboard, and more. Starting with Windows 10 build 18282, you can now choose to apply a light or dark theme for your default Windows mode (system UI) and default app mode (apps) separately or both. Some UWP apps (ex: Microsoft Edge) allow you to change their theme independently of your set default app mode. Starting with Windows 10 build 17666, dark theme support has been added to File Explorer, the File Explorer context menu, as well as the Common File Dialog (aka the Open and Save dialogs). When you make the switch, UWP (Universal Windows Platform) apps including Mail, Calendar and the Windows Store app go to a black background, which can even save your battery power with certain screen technologies.

In Windows 10, you can change your apps instantly from light mode to dark mode – great for low-light conditions like working at night. How to Change Default App Mode and Windows Mode to Light or Dark Theme Color in Windows 10

Dark and light settings