Advanced Technique
Sometimes you may set up a content page that does not show the header on the page. Without your natural header showing, the background on your page may look out of place. That can easily be fixed with some CSS.
An example of a page not showing it's header, but the background still shows:

Hiding the Background
1. Under 'Content' choose 'Add Content' or 'Edit Content'

Hide the header through the 'Show Header and Footer On This Page?' - choose 'Show Footer Only' or 'Don't Show Header or Footer' from the drop down menu

In the 'Custom Header and Footer Javascripts' field, check the box and add the following CSS to the 'Custom Header Javascripts' box:
<style type="text/css">body { background: none !important; }</style>

Apply changes to the page.
And that's it!
Here is the new page:

Changing the Color of the Background
Perhaps instead of removing the background, it is preferable to update the color.
Under 'Content' choose 'Add Content' or 'Edit Content'

Hide the header through the 'Show Header and Footer On This Page?' - choose 'Show Footer Only' or 'Don't Show Header or Footer' from the drop down menu

In the 'Custom Header and Footer Javascripts' field, check the box and add the following CSS to the 'Custom Header Javascripts' box:
<style type="text/css">body { background: #003399 !important; }</style>
*Replace the background color with another hexadecimal color. For example, a white background would look like background: #FFFFFF
Apply changes to the page.
And that's it!
Here is the new page: