Background color disappears when browser width resizes for mobile

Hello,
I have an issue I’m having with the background color disappearing when on mobile.

Here’s an example: on my homepage, http://www.litmos.com, I have a dark color set as the background (#262626) and white header text placed on top. It looks great, unless I narrow my browser to mobile width or view my site on my phone. The text is still present, but the background color disappears, making my text “A Learning Platform for Every Team, Division and Company” effectively invisible. Please advise.

Thank you,
Nancy