Updated de-widthify CSS rules to slightly increase site compatibility.

It's still not perfect, and notably sites that use "!important" in their rules can still override this addon. In the long term some JS is needed to strip such rules from sites (or similar).

* Change icon sizes, in attempt to workaround a possible AMO bug preventing the icon from showing.
* Update description line in source zip to match AMO (likely not user-visible)