Categories in the nav bar

Hello,
i am using devart theme.
i want the nav bar shows the categories instead of pages.

i had change the following code in header

<li <?php if(!is_category()) echo 'class="current_page_item"'; ?>><a href="<?php echo get_option('home'); ?>/"><b>Home</b></a></li>
<?php wp_list_categories('title_li='); ?>

it shows the categories but it dont show the nav-tab for corrent category.

can you help me?

thank you