Those PHP files, that you've mentioned, do not affect this current issue as this is strictly a stylesheet adjustment.
I've just checked the source CSS(current template stylesheet.css) and the declaration is still there, unmodified... it may be that the edit did not upload or the edit occurred elsewhere.
I noticed that you mentioned that you're using "template default". Is that the template override that is currently in your NE_TEMPLATES folder? Or did you edit the actual "template_default" folder's stylesheet.css?
At any rate, I would suggest a template override using a basic template like Classic. I recommend this route because all the relevant stucture is complete. In the 'downloads' area at the top of the forum menu, there is a different colored Classic combo pack available.
http://www.zen-cart.com/index.php?main_page=product_contrib_info&cPath=40_53&products_id=274
You can choose a color version that you like and then tidy it up, instead of trying to refine the stylesheet structure of the default template.