#{resource['/stylesheet.css']}
When using resources from your JSF portlet it's important to ensure that they are accessed in a way that allows the JBoss Portlet Bridge to generic an appropriate Portal URL to the resource being requested.
The correct way to reference a resource is to locate it within your web application within the resources folder so that it the resource can be retrieved using JSF2 Resource Handling:
#{resource['/stylesheet.css']}
stylesheet.css would be present in the root of the resources folder as it does not specify a resource library
Use of #{resource} to retrieve the content is especially import for @import within css files.