Hello,

Usually the custom css is added to customizer -> additional CSS. Now, just to clarify, if you need to add a slight background to the header menu, then you can specify

header {
    background-color: rgba(0,0,0,0.1);
}

with a partial opacity. Or is it for the fullscreen menu?

Hi Bryan,

you should try this one in customizer -> additional CSS

.fullscreen-menu nav.open {
    background-color: rgb(12,12,12) !important;
}

of course, change the rgb component first