You shouldn't be editing the files in the /classic or /template_default folders. If you make your changes there, they'll be overwritten by the next update/upgrade.

Looking at the Zen Cart I have unzipped locally, there is no define_main_page.php in includes/languages/english/html_includes/classic. The original text from includes/languages/english/html_includes/define_main_page.php is

Code:
<a href="http://www.lulu.com/content/466605"><img src="images/large/e-start-book.gif" alt="get your manual today" title="Have you got yours yet? Join the 1000’s of Zen Cart users that have bought the only comprehensive owners manual !" /></a>
<p>This content is located in the file at: <code> /languages/english/html_includes/YOUR_TEMPLATE/define_main_page.php</code></p>
<p>You can quickly edit this content via Admin->Tools->Define Pages Editor, and select define_main_page from the pulldown.</p>
<p><strong>NOTE: Always backup the files in<code> /languages/english/html_includes/your_template</code></strong></p>
I still don't know where .style12 or .style16 is coming from, but REMOVE it entirely and save it in your custom folder. You may have copied it from the html generated by the prototype you did of the layout.