hey, K, thanks for the comment.

it seems like you have this CSS applied to the :hover state of your navigation links:
.nav-menu li:hover > a,
.nav-menu li a:hover {
background-color: #21bbc4;
color: #fff;
}
(not sure why you need li:hover a and li a:hover, but okay)

if that’s the look you want to represent the current page, then you can simply extend the above to:
.nav-menu li:hover > a,
.nav-menu li a:hover,
.nav-menu li.current_page_item a {
background-color: #21bbc4;
color: #fff;
}

that should do the trick.

however, i also see you have this CSS, so you could also eliminate it:
.nav-menu .current_page_item > a,
.nav-menu .current_page_ancestor > a,
.nav-menu .current-menu-item > a,
.nav-menu .current-menu-ancestor > a {
color: #21bbc4;
font-style: none;
}

your call.