I want to tell you how to do it but I may not explain it clearly. I show an image as the background. If you use Firefox and a little add-on with huge assistance, you would be able to see what I'm talking about. So, get Firebug and right click anywhere on your site to find the CSS for it, even in the text boxes for a join form and search form. Select "inspect element" and voila! On the right, it will tell you where the code is located and you can even make changes to see what the change will look like on your site before actually editing a file. If you do this, I know it will help you and tremendously!
With FIREBUG it looks like this for the join backgound:
.input_text, .input_date, .input_area, .input_pass, .input_select, .input_select_couple, .input_select_multi {join.css (line 67)
background-image:url(../images/body_backgr.gif);
border:1px solid #999999;
margin-right:5px;
width:240px;
*****Sorry, can't find my search. Site is being worked on. Hope this helps.