Hello There,
Thanks for writing in! To add a simple bold underline on the active menu state, please add any of the following css code in the customizer, Appearance > Customize > Custom > CSS
.x-navbar .desktop .x-nav > li > a:hover > span,
.x-navbar .desktop .x-nav > li.x-active > a > span,
.x-navbar .desktop .x-nav > li.current-menu-item > a > span {
box-shadow: 0 2px 0 0 #b7b7b7;
}
.x-navbar .desktop .x-nav > li > a:hover,
.x-navbar .desktop .x-nav > .x-active > a,
.x-navbar .desktop .x-nav > .current-menu-item > a {
box-shadow: none;
}
We would loved to know if this has work for you. Thank you.