How do I reduce the width of the theme but keep background full size?

A while back I installed radiate and inadvertently made the theme ‘narrower’ so that it did not take up the entire width of the screen.

Let me try clarify:

On either side of my header and content etc., you could see by background image, but it did not interfere with the text part of my page. It was as though my background image was laid down first, and then a smaller white piece of paper with all of my content was placed over that image. So that my background image was only around the borders of my content page.

I have not been able to reproduce this since I reinstalled wordpress. And I don’t know any coding so that was not how I achieved this.

Please help! I’m so lost that I’ve had to try find another theme which is proving tedious and substandard.