Thank you for your help, but I am not the best at php :/
I placed the first code in custom-functions.php – nothing showed up in Display > Display Navigation (should it be a special position in the code or another file?)
and for the second code; so the line is:
<span class="sup-nav-name"><?php get_theme_mod( 'sup_cat_menu_name' ) ?></span>
?
I use supernova.2.0.61