Customize nav bar – Color

Hi.

I recently started a site on expound. I am trying to change the colors of the nav bar and text. Nav bar to white (ie – invisible) and text to black. then center the text. Ia m not a coder so learning a bit as I go. I put the following into the custom CSS, hit save and nothing changes. Once I get this color solved I will work on centering. Any ideas?? thanks.

/* Change background colour of the entire menu */
.nav-menu {
background: white;
}

/* Change default text colour */
.main-navigation li a {
color: black;
}

/* Change hover text colour */
.main-navigation li a:hover {
color: grey;
}