Change site width without affecting responsive version

Hi There, I've managed to get my site looking satisfactory on the mobile devices but it is too narrow on pc's, mac's etc. I'm sure it's very easy to do. I have been putting in custom CSS into the box in the appearance section on wordpress but when I do this it also changes the view on mobile devices. SO how do I do it without affecting the mobiles? My site is http://www.chartsandscores.com Thank you, you guys are amazing :)