Can\'t change background – Rock Band Theme

Hi, here's my website : marykathill.com I cannot change the background color. Right now, it is black. I would like to change the background and add a photo or several photos to make different section. I have tried background image and colors but nothing changes. It remains black. I also would like to have the photo full width. I don't know why there are two margin on the left and right. I would like to get rid of that black space. Thank you in advance, Mary