Oh, re-reading the OP, I'd like to add one more comment about using DW to edit WordPress themes -- I just realized you are looking at PHP files.
The easier way to do this is simply view the (live) page in your browser, view the source, and copy the entire HTML of the source into Dreamweaver and save it as an html test page (e.g. mytestpage.html) in the root of your local copy of the site. (You will probably need to change the paths referring to your style.css file, etc. -- usually can be accomplished with a simple search & replace, removing the URL portion of the absolute path so that you have a relative path instead.).
That way, you have a static html copy of what WordPress's php is generating. You can now play with the CSS and see what effects it is going to have on the page. If you are planning to change the HTML, you can change it in the test page until you are satisfied, and then edit the actual php of the header/sidebar/content files to generate the new code in the live site.
Hope that makes sense -- to summarize: have WP generate the HTML page, save it as a static page, style the CSS in your editor (and possibly edit the HTML), and then make appropriate changes to the php files of your theme, if necessary.