[HELP!] underline the Current Page title in main menu

Hey all!

I’m building a site for a friend and have came up against a brick wall.

The site uses a mod on this theme: https://www.felixkrusch.de/about

^ As you can see, the current page is always underlined on the main menu.

In the code the menu is displayed by this code:

<li <?php if(is_home()){echo 'class="current_page_item"';}?>><a href="<?php bloginfo('siteurl'); ?>" title="Home">Home</a></li>

<?php wp_list_pages('title_li=&depth=1');?>

and a snippet of css in the stylesheet:

.current_page_item a {

color:#000;

text-decoration: underline;

}

I want to add links to category pages in the menu, but if I add an example like this to the menu:

<a href="blahblah.com/cat/videos">Videos</a>

There is no underline for that page.

Does anyone know a way that I can achieve the same underline effect for non-Pages I add to the menu?

There must be some way to adapt this code for other URLs?

<li <?php if(is_home()){echo 'class="current_page_item"';}?>><a href="<?php bloginfo('siteurl'); ?>" title="Home">Home</a></li>

If anyone can help I’ll be so grateful & glad, thank you in advance!