How to change CBox Background

Login to CBox > Go to Colors & Fonts > Leave Blank to Main BG and Alt BG

In blogger html view, put the following codes.

For Message Background

#cboxmain{
background:url(your image url) no-repeat;
}

For Form Background

#cboxform{
background:url(your image url) no-repeat;
}

Reference: http://html.net/tutorials/css/lesson3.asp