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.
Sheets added via this API take effect on all documents, including
already-loaded ones, immediately.
| Returns true if a style sheet at | sheetURI | has previously been |
| added to the list of style sheets specified by | type | . |
Synchronously loads a style sheet from |sheetURI| and returns the
new style sheet object. Can be used with nsIDOMWindowUtils.addSheet.
Remove the style sheet at |sheetURI| from the list of style sheets
specified by |type|. The removal takes effect immediately, even for
already-loaded documents.