Dewed on "[Theme: Tiny Forge] hide normal menu when mobile menu is shown"

I'm using version 1.5.6 and bending it to my will via child theme on everything I need with one exception so far.

When screen width is shrunk, the mobile menu appears, but the normal menu gets converted to a vertical UL and is still displayed just below it. I'd like to hide it via display:none, but can't figure out how :/

Possibly related, my normal menu is a "custom menu" named top-nav, the resulting HTML looks like this..

<h1 class="menu-toggle"></h1>
<a class="assistive-text" title="Skip to content" href="#content"></a>
<div class="menu-top-nav-container">
    <ul id="menu-top-nav" class="nav-menu">
    <li .......

How can I hide just this top menu when the mobile menu is displayed?

Thanks and keep up the awesome work ! ! ! !