Sorry, there’s no feature in the theme for this. When you view your site on mobile, it’s just un-floating all the columns. So the left sidebar would show up above the main content and the right sidebar would show below. This would be a customization you’d need to setup.
However, I believe this is actually possible in CSS alone with something I learned about recently; it’s called “flex-boxing”.
Here’s an overview: http://net.tutsplus.com/tutorials/html-css-techniques/an-introduction-to-css-flexbox/ (See section titled “Ordering”)
So I think the gist would be you’d target the viewport for mobile, and then set the container around the columns you want to order to
display:box. Then you’d order the elements inside that container (i.e. your sidebar and main content column).