Hi Gemma,
Try the CSS below:
.hd-bar {
background: #fff;
}
.hd-bar .side-menu ul.navigation > li > ul > li > a,
.hd-bar .side-menu ul li ul li ul li a {
color: #555;
}
.hd-bar .side-menu ul li {
border-bottom: 1px solid #efefef;
}
.hd-bar .hd-bar-closer button {
color: #555 !important;
}
@media only screen and (max-width: 767px) {
.hd-bar .side-menu ul li a {
color: #555;
}
}
Copy and paste it in Appearance -> Customize -> Additonal CSS
Let us know if you have any questions
Best Regards