Welcome to the JavaRanch, Raphael!
I have doubts that your EL in your CSS file really works. At least if it's a real CSS file retrieved by a real URL.
Neither CSS nor JavaScript files should be processing EL for the simple reason that they don't pass through an EL processor. In modern-day
JEE, there's a Unified EL subsystem and it will deal with EL in JSF View Definition Language and with plain JSPs, but it isn't applied to non-JEE resources. In part because running EL on something like a binary image file would be a disaster!
Your CSS also has another issue. The FacesContext is not a permanent or long-term object. It exists
only for the duration of the request process for the URL. If that URL wasn't routed to the FacesServlet, the FacesContext won't exist for that request.
So the only way you could get EL to work in either js or css files would be to brute-force include them as part of the source of a JSF page. If they're fetched as distinct URLs (such as via the <LINK> element, they won't go through the EL processor.
Last, but not least, there's the addressability issue. JavaScript runs on the client. Your properties live on the server. So it takes more than just EL if you want general access to them.
The outputScript and outputStyleSheet elements are new to JSF2 and the documentation I've found for them is pretty vile and looks to be more for the benefit of the tag implementor than it is for people who are actually using the tag. But as far as I'm aware, these elements do NOT route their respective resources through the FacesServlet, they just generate JSF-friendly HTML. Primarily that means that the output URLs know about the
servlet context path without making you supply it explicitly.