How to add underline on main menu when the item is active?

Hi there,

There isn't underline added by default so I'd assume you've added some CSS for it.

I'd need to see the site in question to be able to help.

I updated the question

Try this CSS:

.main-navigation .main-nav ul li[class*="current-menu-"] > a {
    text-decoration: underline;
}

Adding CSS: https://docs.generatepress.com/article/adding-css/

Now, how do I remove it from the submenu?

Try this instead:

.main-navigation .main-nav > ul > li[class*="current-menu-"] > a {
    text-decoration: underline;
}

Thank you!!!!

No problem :)