Hi,
That comes from the CSS:
.overlay-light a, .overlay-light .g-main-nav .g-toplevel > li:hover > .g-menu-item-container, .overlay-light #g-navigation .g-main-nav .g-toplevel > li.active > .g-menu-item-container {
color: #fff !important;
}
in the file templates/rt_orion/scss/orion/styles/_utilities.scss of your template.
So I'm not sure what to do. As it might affect other areas of your pages and since there is an !important flag, it can't be easily overridden either.
I would recommend to check with your template provider since it's something added by them.