how to make overall content area narrow

I am new to WP and code in general so I’m learning as I go. I am using the tempera theme with a child them.

My site is at http://www.astralingua.com/makeitso

I would like to narrow the overall width of my content area so that I can see more of my background picture on the left and right.

At the moment I am using empty sidebars in my theme to narrow the text/content of each page. But I would like to get rid of the black background in those sidebars. That way I have a black background directly behind the text but not as far out on each side. How do I do this?

If I turn off the sidebars right now the width of the content (black) area stays the same and the words/content gets wider. How do I narrow the entire thing?

Thanks!