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

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Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>