Decrease Size of Main Content/Container (without sidebars)

Hello,

Can someone tell me how I can decrease the width of my content/container area on my pages? I want to set a background image and allow it to be more visible on the sides by narrowing my content area. I do not want there to be sidebars, so I have been working with the “full width mode” template.

I can’t figure out how to decrease the container width, while keeping the entire thing centered on the page. Help please!

Lindsi