This seems like a pretty typical use case, but I'll be damned if I can figure it out. There must be something fundamental I'm missing.
find-dired is basically a front end to the find command that yields a dired-buffer of results
find-grep-dired seems to be more of a front end to grep more so than find. and yields a dired-buffer but it searches all files in the directory tree.
What I would like is the ability to start at a given path and search *.css for #some-id and have it give me a dired-buffer for further processing.
Seems all the pieces are there but I haven't figured it out. So I'm thinking it's something fundamental I may have missed.