https://docs.microsoft.com/en-us/windows/win32/api/dwmapi/ne-dwmapi-dwm_systembackdrop_type?WT.mc_id=WD-MVP-5002756

Mica lives inside the Desktop Window Manager (DWM) and a new Windows 11 variable called “DWM_SYSTEMBACKDROP_TYPE” will allow developers to specify Mica or Acrylic material in their desktop apps.

There are four constants that developers can choose from:

DWMSBT_AUTO: This is the default behaviour. In this constant, Desktop Window Manager (DWM) will automatically decide the system-drawn backdrop material for the app’s window.
DWMSBT_NONE: Developers can skip Mica or Acrylic if they’ve their custom design for the app window. For example, Spotify comes with its own header. When this value is set, Microsoft won’t don’t draw any system backdrop.
DWMSBT_MAINWINDOW: Applies the backdrop material effect corresponding to a long-lived window.
DWMSBT_TRANSIENTWINDOW: Applies the backdrop material effect corresponding to a transient window.
DWMSBT_TABBEDWINDOW: Applies the backdrop material effect corresponding to a window with a tabbed title bar.