Additive CSS for workspaces; how to?

I want to provide custom CSS for workspaces that generally follows the main wiki CSS, but adds a small number of rules.

So I would like to have workspaces load the same CSS file as the main wiki, plus an additional CSS file custom to each workspace.

1. It appears that supplying a custom CSS file for a workspace causes that workspace to load only that CSS file, and not the wiki's CSS file. That's understandable, though not what I want. I could duplicate the wiki CSS in each workspace's file, but that's a maintenance headache each time the main CSS gets revised.

2. I tried using @import in the workspace CSS to load the main wiki CSS file. But it seems that the CSS uploader strips out the @import statement.

3. On the basis that I could auto-generate the workspace CSS files and store them somewhere, I tried the "Enter a web URL" feature of the Custom CSS settings, but I couldn't get it to do anything. Actually, I'm not sure whether it's supposed to fetch a CSS file once from that URL, or is it supposed to put that URL for CSS into pages each time? Anyhow, it didn't seem to accept any URL form I tried.

Bottom line: what options do I have for implementing workspace custom CSS that's only slightly different than the wiki CSS?

Thanks,

- Graham