CSS::-moz-system-metric(windows-default-theme)
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This article covers features introduced in Firefox 3
[edit] Summary
:-moz-system-metric(windows-default-theme) will match an element if the user is currently using one of the following themes in Windows: Luna, Royale, Zune, or Aero (i.e., Vista Basic, Vista Standard, or Aero Glass). This will exclude Windows Classic themes as well as third-party themes.
This selector is intended for use mainly by theme developers.