Adjust mobile sidebar width

Hello,
I’m using the Twenty Twelve theme v1.9 on WP 4.4 with a child theme on the following website: youputiton.com. On mobile, the sidebar appears below the main content, which I’m fine with. However, I’d like to increase the width of the sidebar to the full width of the device. Right now there’s just empty space to the side of the sidebar. I’ve tried looking for the code but cannot seem to effect the width when I look at the site on my phone. Thanks for any help you can provide!