If there aren't too many styles sheets, then I'd open the site up in a browser, then view source and find the path to the CSS file(s) and then copy and paste that location into the browser address field. If you're looking at example.com and that page references css/styles.css, then I'd enter example.com/css/styles.css into the address bar, and save that file locally.
Of course, if there are any background images referenced, you'd also need to build their URLs and download the files, which could be an issue.
But it may be as simple as using Site Import to download the "site", but set the limit of pages to 1, and that should pull all the assets for that page.
