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 /sdcard/html/myfile.html
:
<link rel="stylesheet" type="text/css" href="style.css">
: simply copy style.css
to /sdcard/html
href="../style.css">
: Copy to /sdcard
href="foo/bar/style.css"
: create /sdcard/html/foo/bar
and place style.css
there
href="/style.css"
: Sorry, nogo without root, as you cannot write to /