I recommend getting rid of the following code in custom.css:
#s5_center_area2 {
width: 85% !important;}
The site width can be defined in the theme options, and if you want to set a "max-width", to keep it from being too wide, that's in there too. Before hard-coding any layout CSS, you should always make sure you check to see if what you want to do is handled by the theme options; especially if it's related to the responsive layout, as there are multiple things that need adjustment to keep the site responsive, and the framework is designed to make sure that is done correctly.