Change sidebar position order upon responsive page resizing

I want to use the left sidebar for faceted search filters, like when sorting out a list of products by features. The main content page section will contain the list of products, and the left sidebar will contain the product features to use when filtering.

On mobile devices or when the screen is resized narrow, I want the left sidebar to appear ABOVE the main content, so the filters are above the product list.

Customizr currently puts the left sidebar BELOW the main content section. I assume they are defined somewhere either in html or css to be 100% width when page width falls below a certain number of pixels.

So how do I reverse the order of the page sections when the page is made narrower?