Changing navigation underline weight

Hi SS Circle trust,

How can I adjust the line weight of the underline on current state navigation links? Have tried some older code, but no success.

Site: lettuce-hexagon-lw6p.squarespace.com
PW: Ruello2023!

Thank you in advance!
Rad. 🙏

1 hour ago, RAD_Design said:

Hi SS Circle trust,

How can I adjust the line weight of the underline on current state navigation links? Have tried some older code, but no success.

Site: lettuce-hexagon-lw6p.squarespace.com
PW: Ruello123!

Thank you in advance!
Rad. 🙏

Wrong password, Can you check it again?

Edited in the original post. Sorry!

My testing

Thank you @Beyondspace this worked perfectly on Desktop!

Any idea how I would adjust for mobile view also? 

Cheers!
RAD. 🙏

Use this code for mobile

.header-menu-nav-item--active .header-menu-nav-item-content {
    background-size: 1px 4px !Important;
}