Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Summary
:-moz-system-metric(mac-graphite-theme) will match an element if the user has chosen the "Graphite" appearance in the "Appearance" prefpane of the Mac OS X System Preferences.
This selector is intended for use mainly by theme developers.