Change the background color & height of the nav menus

You can change both in the style sheets

Changes should NOT be made to theme files – as they will be lost when the theme is updated. Instead use Custom CSS or a child theme.

That green might be some kind of theme option – I’m not familiar with this theme. But if it’s not, try adding this to custom CSS if your theme has that option otherwise in JetPack’s custom CSS under Edit CSS:

#nav li:hover, #nav li.sfHover, #nav li:hover > a, #nav a:focus, #nav a:hover, #nav a:active, #nav li li a {
    background: none repeat scroll 0 0 #75AB21;
}
#nav li ul li:hover, #nav li ul li.sfHover, #nav li ul li a:focus, #nav li ul li a:hover, #nav li ul li a:active, #nav li.current_page_item > a, #nav li.current-menu-item > a, #nav li.current-cat > a {
    background: none repeat scroll 0 0 #75AB21;
}

Change the color code in the above.

To change the height, add this CSS – and change the padding as you like:

#nav a {
    padding: 1.7em 1.7em;
}