There are two ways to set a page background for the pages within your website.

To apply a background globally to all your pages, galleries and events:
Website Controls > Manage Website Design > Content Window > Content Window Background

For a single page:
Website Controls > Manage Pages and Galleries > Click the Edit button beneath the appropriate page, gallery or event >
Page/Gallery Design Tab > Page/Gallery Background
This will override the global font settings above.

If a page, gallery or event has no background set and no background has been set for the Content Window as well, then the Website Background will show through. In other words, the Content Window will be transparent. This does not apply to older versions of Internet Explorer which will always set a page background to white if no other has been set.

Related article: Working with page fonts