help with custom background

Hello, I recently created a website for a client using the wordpress twenty twelve theme as a template and then created my own child to incorporate a custom style sheet. Here is a link: http://www.militarytracks.com. I’m hoping someone can help me maneuver the border around the website to look a big more professional. Currently I have the image uploaded as a background in the appearance editor but I know there’s a better way to do that but I’m not sure how. I’m not sure if I’m supposed to update the style.css or header.php file, or maybe neither of those? I’d also like to remove the white line above and below the navigation bar as well. Any support would be great! Thanks,