Not responsive on iPhone 6 and iPhone 6 Plus

Hi! With the release of the new iPhones my site is no longer perfectly optimized in a responsive sense, for said iPhones ofc. my site: www.obsid.se I want the site to be edge to edge, no background image when on a phone. It worked perfectly on all iPhones from 3 to 5s, but when visiting with the new screen sizes I get a nasty background-image-frame which makes the site look like it's from 2003. If I've understood this correctly, all I need is some CSS-code that I paste in the "Custom CSS-box", right? If that is the case, can anyone skilled in CSS please post the necessary CSS to fix this? I would appreciate it immensely. Thanks in advance and greetings from Sweden!