Add class to WordPress admin bar?

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There isn’t a filter/wrapper for the admin bar CSS that you can plug into , the function is simply 

  function wp_admin_bar_header() { ?>
  <style type="text/css" media="print">#wpadminbar { display:none; }</style>

You can over ride the CSS using !important or use jQuery to prepend a class/id to the bar.

Something like:

jQuery("<div id='custom-width'></div>").prependTo("#wpadminbar");