Highlight Current Category Menu Item for WordPress Single Post

- Add to funtions.php

function sgr_show_current_cat_on_single($output) {
     global $post;
     if( is_single() ) {
          $categories = wp_get_post_categories($post->ID);
          foreach( $categories as $catid ) {
   $cat = get_category($catid);

        // Find cat-item-ID in the string
        if(preg_match('#cat-item-' . $cat->cat_ID . '#', $output)) {
             $output = str_replace('cat-item-'.$cat->cat_ID, 'cat-item-'.$cat->cat_ID . ' current-cat', $output);
        }
          }

     }
     return $output;
}

add_filter('wp_list_categories', 'sgr_show_current_cat_on_single');

- Add style.php 

.widget ul li.current-cat {
background-color: #26609b;
font-weight: bold;
}