styling fullwidth submenu active item

I’ve built a page with a Fullwidth Sub Menu, and the menu items are links to anchors in the content below, on the same page.

I’m able to style the submenu items on :hover, but how can I get the active menu item to be styled differently than the rest (different color and background-color), when the page is scrolled to that anchor point, whether by clicking on the submenu item or just by scrolling? Already tried the following, and no luck:

#top .av-subnav-menu > li:active {
background-color: #666666;
color: #ffffff;
}

Thanks!