Well, from what I can see, XML format is similar to CSS and HTML, and if I'm not mistaken, I could just go through the code, find the source images (like the header at the top) and then switch the URL.

Also, with a Google search I found this: http://www.vbulletin.org/forum/forumdisplay.php?f=247
Dozens of VB4 themes to choose from, and I'm sure there are some similar to the "Planet Earth" theme VB3 had.