style: Defer to -moz-default-appearance when appearance is auto.
style: Defer to -moz-default-appearance when appearance is auto.

Differential Revision: https://phabricator.services.mozilla.com/D83429

KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo