You will have to use custom css for that, try this changing the hex values as you desire
@media ( max-width: 768px ) {
.main-navigation {
background-color: #222222;
}
.main-navigation .main-nav ul li a,
.main-navigation .menu-toggle {
color: #FFFFFF;
}
.main-navigation .main-nav ul li > a:hover,
.main-navigation .main-nav ul li > a:focus,
.main-navigation .main-nav ul li.sfHover > a {
color: #FFFFFF;
background-color: #1e72bd;
}
}
https://gist.github.com/generatepress/793804f553c25c70823c