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If you want to fiddle with the theme to try and get it the same as the XOOPS one (or any other) your first step should be to set up a XOOPS site on your own comp so you can play around without destroying your live site.
All the files you need to change would be in the /themes/default folder, you just have to change various things in style.css to change how a page is displayed for that particular theme. If you have or can get a CSS editor like Top Style Pro, it makes things all the easier. You may also have to change some things in theme.html, depending on what you want to achieve.
Once you have things the way you want, you can either upload the default theme from your comp and overwrite the original on your live site, or rename it to something else.
I'm in the process of doing exactly what you ant to do, except i'm modifying a different theme. If I've got anything wrong in above or anybody can suggest anything else, please feel free.
Oh, and with the banners, you'll need a good graphics program such as Adobe Photoshop, and perhaps something like Ulead Gif Animator or Macromedia Flash if you want to make animated banners. Photoshop and Flash are very expensive, but there are other cheaper/free alternatives out there.
Good luck :)