Apologies for the delayed response. If you’re talking about the class completion described here: https://github.com/Zignd/HTML-CSS-Class-Completion then you should actually be seeing classes from across your workspace, not just the current project and its references. I’m afraid the referenced projects aren’t really taken into account when making suggestions (and you would get suggestions from all projects).
If you’re not getting suggestions from the WebResources project, it’s possible that something else is wrong with this particular project – are the classes in .css files or elsewhere? Do you see references from other projects?