Mica is Microsoft’s new transparency effect for Windows 11. It’s similar to Acrylic but optimized for performance by only sampling from your background image and not the app underneath you.
Microsoft has rolled out the new look to more and more native apps, and now Office Insiders and Beta users are reporting that the effect is showing up in their apps as well.
As can be seen from the screenshot above (posted on Reddit) The colorful bands of the Windows 11 wallpaper can be seen through the new dark mode of office applications.
Microsoft has updated apps like Chromium Edge, Movies & TV, MS Paint, Snipping Tool, Photos, Calculator and others with Mica effect and is expected to bring it to the default title bar and other chrome of UWP applications.
https://mspoweruser.com/microsoft-is-testing-mica-transparency-effects-in-office-apps/ Microsoft is testing Mica’s transparency effect in Office apps