This line loads the extra style definition:
* Set USERSTYLEURL = %ATTACHURL%/centerpageborder.css
You can write this line in
TWikiPreferences, in
WebPreferences, in your user topic or on a single page.

If you don't write this overloading style in a template and use an external
.css
file, you need to set the image to the absolute url:
<style type="text/css" media="all">
html body {
background-image:url("/asterix/pub/TWiki/PatternSkin/gradient_page.gif");
background-repeat:repeat-x;
}
</style>
You can always write a
<style>
in a topic - all current browsers support this - but the page won't validate as valid XHTML.