nsIStyleSheetService Interface

 Synchronously loads a style sheet from |sheetURI| and adds it to the list
 of user or agent style sheets.

 A user sheet loaded via this API will come before userContent.css and
 userChrome.css in the cascade (so the rules in it will have lower
 precedence than rules in those sheets).

 An agent sheet loaded via this API will come after ua.css in the cascade
 (so the rules in it will have higher precedence than rules in ua.css).

 The relative ordering of two user or two agent sheets loaded via
 this API is undefined.

void loadAndRegisterSheet(in nsIURI sheetURI, in unsigned long type)

 Returns true if a style sheet at |sheetURI| has previously been
 added to the list of style sheets specified by |type|.

boolean sheetRegistered(in nsIURI sheetURI, in unsigned long type)

 Remove the style sheet at |sheetURI| from the list of style
 sheets specified by |type|.  All documents loaded after
 this call will no longer use the style sheet.

void unregisterSheet(in nsIURI sheetURI, in unsigned long type)