very similar approach to the answer by @AndrettiMilas:
add_filter('wp_list_categories','style_current_cat_single_post');
// filter to add the .current-cat class to categories list in single post
function style_current_cat_single_post($output) {
if( is_single() ) :
global $post;
foreach ( get_the_category($post->ID) as $cat ) {
$cats[] = $cat->term_id;
}
foreach($cats as $value) {
if(preg_match('#item-' . $value . '">#', $output)) {
$output = str_replace('item-' . $value . '">', 'item-' . $value . ' current-cat">', $output);
}
}
endif;
return $output;
}
adapted from one of my articles.