You should not do this via CSS as the underlying functionality would still be there (and thus open for direct access).
I’d rather rewrite the theme in a way that the menu pages are only added if the current user has a certain capability.
if (current_user_can( SOME_CAP )) {
add menu pages ...
}
You could even provide a new capability for your theme. Have a look over here, if you’re interested in this.