That's normal, as LSWS now only process matching context when there is no explicit direct match. So, in your case, the matching context has no effect. Normal contexts have high priority than matching contexts.
With current release, you have to use normal contexts for those files like /css/ you tried.
There is another possible work around by avoiding using FastCGI Context '/' which will match any request, instead, define a Matching FastCGI context with URI "exp:^/$" to match only request to '/' and other FastCGI contexts for each unique URI that should be processed by the FCGI.
But, there is a bug with FastCGI Matching Context in current release that will be fixed. So you have to wait till next release to be able to do that.