andreas00 - works without images too!
While it doesn't look very pretty, the andreas00 template works well even without the included background images. This page shows an example of what the template can look like without images and with some minor additions to the stylesheet: reduced size of the header and a small padding for the body.
It is of course also perfectly possible to add your own images. Open source website templates can work both as complete websites (if you like the way they look by default) or as starting points (if you want to add your own styles to it).
Useful tools
To edit a template like this one, all you really need is a text editor. There are many to choose from, no matter which operating system you use. As a Windows XP user I can recommend Notepad++, an amazing freeware code editor. If you look for more specialized CSS design programs I can warmly recommend Style Master, a truly great CSS editor which is available for both Windows and Mac OS X.