Nav text "underline"

Hello,

By default, the header has the underline option when the link is “hover” or active, great. My client wanted a larger and closer underline, what I’ve did with :
`.avia-menu-fx {
height: 6px;
bottom: 20%;
}`

But when you scroll in the page, the bar is too close from the text in the sticky header reduced, and I don’t see the css difference between the normal nav and sticky reduced nav. Could you help?
Tx !
Marine