Show category-ID in custom category-list

Try this:

<div id="category-list">
    <ul class="topnav"> 
        <?php
        if (is_single()){
            global $post;
            $pid = $post->ID;
            $post_categories = wp_get_post_categories( $post->ID );
            $cats = array();
            foreach($post_categories as $c){
                $cat = get_category( $c );
                $cats[] = $cat->ID;
            }
            $active_cat_count = true;
            $cat_count = 0;
        }

        $kategorien = get_categories(array(
        'child_of' => 0,
        'exclude_tree' => 15,
        'exclude' => 16,
        'orderby' => 'slug',
        'order' => 'ASC')); 
            foreach($kategorien as $kategorie) { 
            if (isset($active_cat_count) && $active_cat_count){
                if (in_array($kategorie->ID,$cats){
                    $active_cat_count = false;
                }else{
                    $cat_count = $cat_count + 1;
                }
            }
            echo '<li><a href="#" class="category-title">'; echo ($kategorie->name); echo '</a>';
                echo '<ul>';
                    $cat = $kategorie->cat_ID;
                    $posts = get_posts('category='. $cat .'&posts_per_page=-1');
                        foreach($posts as $post) { ?>
                        <li>
                            <a <?php if ($post->ID == $pid){echo 'class="current"';} ?> href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
                        <li>
                    <? }
                echo '</ul>';
        echo '</li>';
            } 
        ?>                  
    </ul>
</div>
<?php
if (is_single()){
?>
<script>
jQuery(document).ready(function(){
    $( ".topnav" ).accordion( "option", "active", <?php echo $cat_count; ?> );
});
</script>
<?php
}