Adding a background-image rule via the CSSOM

To add a background-image rule via the CSSOM, first get a reference to the rules of the first stylesheet:

var stylesheet = document.styleSheets[0].cssRules;

Then, get a reference to the end of the stylesheet:

var end = stylesheet.length - 1;

Finally, insert a background-image rule for the body element at the end of the stylesheet:

stylesheet.insertRule("body { background-image: url('http://cdn.sstatic.net/Sites/stackoverflow/img/favicon.ico'); }", end);