Ah ok.
So in Customizer > Additional CSS look for this:
.main-navigation .inside-navigation .menu .slideout-toggle,
button.menu-toggle,
.slideout-overlay {
display: none !important;
}
And change it to:
.main-navigation .inside-navigation .menu .slideout-toggle,
button.menu-toggle,
.slideout-overlay .slideout-exit {
display: none !important;
}
Then if you want to change the overlay color from default then use this CSS:
.slideout-overlay {
background-color: rgba(255, 255, 255, 0.6);
}