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. The system UI includes the taskbar, Start menu, Action Center, touch keyboard, and more. As part of this work Microsoft is also adding a new default wallpaper. Selecting the new Windows Light theme will also change your PC to be light themed.
This tutorial will show you how to add a Windows mode context menu to make it easy to choose the default Windows mode to a light or dark theme as wanted for your account in Windows 10.