Google has a clean and simple background interface for its homepage. But if you like to change the default background of google.com and put a colorful background as in Bing, a Greasemokey script could help you do that. Just follow the below steps.
1. First install the Greasemonkey addon in Firefox and restart the browser. If you already have it, then proceed.
2. Install this script using greasemonkey and you’re done!
NOTE: If you want to change the background image, just replace the image URL with the existing one.