How can I change background image after x minutes of mouse inactivity?

I can not find anything on the subject. What I am hoping to do is to replace the HTML/CSS background image after a period of mouse/keyboard inactivity, or replace the whole HTML page. There is a "onmousemove" but nothing like "onmousestill". Maybe I am not "googling" with the right keywords.

Can anyone make a suggestion? CSS or Javascript?

Thank you for your help.