There are no images direct in the source for most Q2A themes, they are usually all background images in the CSS. So for that you can change the CSS to point to the CDN very easily.
For CSS files, you could check the $css_src variable inside the head_css() theme function and modify if necessary.
It would be a slightly easier to make a core hack of those lines in qa-page.php to put your CDN URL in there.
I'll see if I can think of a way to make this easier in a future version of Q2A.