2012 theme menu links turn white when viewed on mobile device

Hello, I’m troubleshooting a site that uses a 2012 child theme. When looking at it on a mobile device (or < than 600 pixels in width) and you click the menu button, it stacks properly but the the link text turns white so you can’t see the links.

Here’s the site: http://hilltron.com/ and also a screenshot of what I’m looking at: http://hilltron.com/wp-content/uploads/2013/09/screenshot.jpg

Any tips for fixing this?

Thanks!