Page 141 - CSS

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);

Read CSS Object Model (CSSOM) online: https://riptutorial.com/css/topic/4961/css-object-model--
cssom-

https://riptutorial.com/ 119