Mica is Microsoft’s new transparency effect for Windows 11. It is similar to Acrylic but optimised for performance by only sampling from your wallpaper rather than the app below you.
Microsoft has been rolling out the new look to more and more native apps, and now Office Insiders and Beta users are reporting that the effect is also showing up in their apps.
As can be seen from the screenshot above (posted on Reddit) the colourful bands of the Windows 11 wallpaper can be seen peeking through the new dark mode of the office apps.
Microsoft has already updated apps such as Chromium Edge, Movies & TV, MS Paint, Snipping Tool, Photos, Calculator and other apps with the Mica effect, and is expected to bring it to the default title bar and other chrome of UWP apps.