Use inspect elements or, view page source and then direct to the global.css file. Once in there, copy the code and paste it into a text editor such as notepad++ and add the website/forums url (http://example.com) to the beginning on the paths to the images.
So, if you copy something like this:
body { color: fff; background: url(.../.../.../images/theme/background.jpg) no-repeat; }
a { color: eeeeee; text-decoration: none; }
You'd edit the background of the body tag and replace the periods with the websites url, so it'd end up like this: