If you’d like to change the background color of pages in the Extra Theme, you need to use some custom CSS.

For example, you can make all pages have a red background, you can use the following CSS:

/* Change background color for all pages */
.page article { background-color: red; }

You can add this to the theme by pasting it into the “Custom CSS” box found in the ePanel at “Extra > Theme Options > General Settings” (at the very bottom).

If you’d prefer to change the color on a single page, you can instead do the following:

/* Change page background only for page with id 88 */
article#post-88 { background-color: red; }

For this you need to know the page’s id. You can find this in a number of places. For instance, it is part of the URL shown in the address bar when you are on the page edit screen (note that it isn’t shown when you first create a new post – you need to save it as a draft before the id shows up).