Hide active link underline in custom fullscreen menu

Hi, 

I am hoping the Squarespace faithful can help me. 

Website: www.ah.studio
Password: ahs23

I have built a custom fullscreen menu for my website. However, for active links, I wondered if there was a way of removing the underline in the menu? I've tried doing this, but it's not having any impact:

div.header-menu-nav-item--active a {
   opacity: 0.3 !important;
  text-decoration: none !important;
}

Can anyone tell me what I'm doing wrong?

Thank you very much in advance! 

@Lesum Thank you so much, worked perfectly!! Really appreciate your expertise. 🤝