Change the underline of the active mobile nav item not working

Client website is still on trial so can't share the URL.

I'm trying to change the underline of the active mobile nav item but the code doesn't seem to effect the underline. I have successfully changed the colour, but not the underline.

.header-menu-nav-item--active a {
color: #A4804A !important; <----- THIS PART WORKS
text-decoration: underline wavy #A4804A !important; <----- THIS PART IS NOT WORKING
}

The code I used above worked on the desktop navigation fine but doesn't seem to respond on mobile..

If the site is trial, you can setup an access password & share url