Error: nsIDOMCSSStyleSheet.getCssRules() from xulrunner in designmode

I am working on an Editor based on xulrunner 1.9.0.5(running as a
Eclipse plugin) from javaxpcom to create an Eclipse Plugin.
I can get hold of a nsIDOMCSSRule (from e.g. inIDOMUtils and a
nsIDOMNode) and manipulate the Css, but I cannot find a way to get a
nsIDOMCSSRuleList to manipulate a nsIDOMCSSRule directly. I understand
there is a javascript security issue, but in "designmode" javascript
seems to disabled anyway.

To call nsIDOMCSSStyleSheet.getCssRules() really seems important for
my app, in order to update any Css manipulation directly, without
reloading the document.

Thanks any help!

Andreas Talita