This can happen when content overflows its container element. I could see a black bar to the right, that appeared on smaller screen sizes. I wasn’t sure where the left black bar was. But I’ve created a video that explains how to fix this kind of problem: http://somup.com/crle6Y0ycR
In short, go to your Post 2 entry content selector in the General folder. Then go to the Dimensions property group and set the box-sizing property to border-box.
This may also solve your left black bar, but let me know if not.