Change color behind text only (not background image), Twenty Eleven

Did you try changing the background color for p instead of #page in the CSS?

I struggled with the same problem, but changed the text to white, with the Widescreen theme :

At the expense of writing a long explanation, let me know if this is the sort of thing your looking for and I’ll re-post.

Here’s the site in question
https://www.luxphotography.com.au

Try using a browser developer tool for this kind of exploratory CSS work.
I used Google Chrome’s built-in Developer Tool and found adding overflow: hidden; to the <div id="page"> element resolved the issue [screenshot].

Now that you know the style, from being exposed/added by the Developer Tool, you can add a new style to override it.

You ought to add new styles externally, not within the theme’s files. Changes to the theme’s files will compromise the theme and be compromised when the theme updates.

External changes can be made using a Child Theme or through a plugin that provides Custom CSS functionality.

I recommend using the Jetpack plugin and its Custom CSS functionality to hold your CSS modifications.