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function admin_css() {
 echo '<style>
   .focus-on #wpcontent {
    margin-left:0;
    width:100%;
   }
.  .focus-on ##wpbody-content .metabox-holder{
    width:100%;
   }
   .focus-on #poststuff #post-body.columns-2 {
       margin-right:20px;
   }
   .focus-on .wp-editor-expand div.mce-toolbar-grp,
   .focus-on .wp-editor-expand #wp-content-editor-tools {
       width:100% !important;
   }
 </style>';
}
add_action('admin_head', 'admin_css');