Microsoft has started rolling out new Fluent-based icons for Windows 10 apps and system applications to Insiders on the Fast Ring. Microsoft has modernized their icons to contain more depth and color so that they are recognizable among both mobile and desktop operating systems. The new icons will be phased in over the next couple of months through app updates via the Microsoft Store and via release previews of Windows 10. As with all Microsoft rollouts, not everyone will see these new icons at the same time.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-unveils-their-new-windows-10-system-icons/