4
I don't use the default theme, but here's what you'll probably have to do:
Make Backups First!
1. Open the "themes" folder
2. Open the "default" folder
3. Open "style.css"
4. Add this line to the line that starts with "body"
background-image:url('images/yourimage.gif');}
You'll need to change the "yourimage.gif" to reflect your image's filename. I'd also put the file in the Default Theme's "images" folder. Any changes made to the style.css should also be copied to styleMAC.css and styleNN.css so that the site looks consistent in Firefox and on Macs.
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I notice that the default theme also has a css folder. Don't know what the heck that's all about, but if the changes don't work for you altering the style.css in the main theme folder, you may have to monkey around with the style sheets in the "css" folder also.