Thank you Mauno,
I need to change the background of category list or course list which we have seen after logging.
*( what I mean, this will appear, if a role(or course) never assigned to a user(or student)). Please check once.
I tried by changing the respected theme *.css files. But I can't get it. How can we change this background?
(By viewing the view source in browser the background tag defined as In-line style. Where can we find the code for this page?)