So you would like to display the current nav link a different colour to show it's active. You can do that by editing this code:

<?php if( $item->title[$default_lang] == strip_tags($page_title) ){?> active<?php }elseif( $item->uri == '/' && $page_title == "Welcome" ){ ?> active<?php } ?>

So you need to update the Welcome with the title of your index. and it should work.