I don't know much about Eclipse. But for your goal, it all depends on how the external stylesheets are defined. If they are using full-qualified URLs (e.g. http://www.example.com/example.css), there's no way you get them working offline. If they are using a relative path (easiest: just their file name, residing in the same directory as the HTML file), you can simply copy them next/relative to your HTML files, and it should work.
Examples, assuming the HTML file is put to
<link rel="stylesheet" type="text/css" href="style.css">: simply copy
href="../style.css">: Copy to
/sdcard/html/foo/bar and place
href="/style.css": Sorry, nogo without root, as you cannot write to