You already have Mica and Mica Alt in file explorer, notepad...
Mica Alt is normally used in Title Bars.
And I think it's only available for developing apps and not for customizing windows.
I don't have Notepad app with mica background, but it appears on the microsoft website, as you can see in the link below.
Introducing a big update to Windows 11 making the everyday easier including bringing the new AI-powered Bing to the taskbar