How can I override the menu toggle on AdaptiveTheme?
Oddly enough I haven't really seen this issue addressed anywhere else.  I found that the most straightforward solution is to put this in your theme's template.php and then style the span tag:

function your_theme_preprocess_block(&amp;$variables) {
  if ($variables['block_html_id'] == "block-system-main-menu") {
   $variables['title'] = "&lt;span id='menu-toggle-button-icon'&gt;&lt;/span&gt;";
  }
}


Cheers