Following #41842, this is an extension to that contribution and it enables `MicaBackdrop` and `MicaAltBackdrop` for `"background.appearance"` Release Notes: - Add `MicaBackdrop` and `MicaAltBackdrop` to be selectable on themes Additional Notes: If PR is merged, theme schema should be updated to 0.2.1 or higher --------- Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>