Effect in navigation bar menu

I user Grav + Gantry and the Helium theme.
How can I replicate the same effect of the navigation bar menu in this site?
https://demo.getgrav.org/grav/

You mean the underline of the menu item when you hover it?

The bar menu is fixed at the top of the page and when you scrool down, the bar menu resize.
I want replicate this effect

Pretty sure this 'fixed' header question is answered here: https://github.com/gantry/gantry5/issues/390

I don't want a fixed menu, but resizing menu, like this http://callmenick.com/_development/resize-header-on-scroll/