Fix CSS for prod builds.

[rwjblue]

See glimmerjs/glimmer-application-pipeline#74 (comment).

Basic gist:

class CustomApp extends GlimmerApp {
 cssTree() {
   let cssTree = super();
   return customBroccoliStuff(cssTree);
 }
}
let app = new CustomApp(defaults);
return app.toTree();

[cafreeman]

@rwjblue I've got this working on my other app now, so I'll fix it here as well. Expect a PR soon. :)

[mmun]

Should this be closed?