That is a basic usage of the Background CSS tag (the URL to documentation was supplied with Isaac's last post); to take care not to completely hose your style, you should have a solid understanding of CSS before continuing with the Style Editor.

The General CSS -> Body class background:
Code
background:#101012 url('../styles/images/chalkboard.jpg') fixed !important;
That particular instance of the tag is:
Defining a color of #101012
Embedding an image to the background: url('../styles/images/chalkboard.jpg')
Defining that the background should be fixed on the page
!important means to override any other specification

Again, this is all included on the W3C documentation.

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