Go to your CSS in your FTP. Go to the file: www.example.org/forum/templates/default/style.css

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Edit: Link changed according to RFC 2606, Section 3. Change www.example.org to the domain of your site. [Alfie]
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Search for:
#top { margin:0; padding:0; height:4.4em; color:#000000; background:#d2ddea url(images/bg_top.png) repeat-x bottom; }

The bold part. That is at the header, which you want to change. Additionally you can select another image by overriding it or hosting it externally using ImageShack.us or make it into another color, such as pink.

#top { margin:0; padding:0; height:4.4em; color:#000000; bgcolor:#hexcode; }

Change the hex code to your color. But sometimes gradiants look better, but then again one color headers can be used efficently as well. If you need any more help add me on MSN: blackaye_v2@hotmail.com or go to my contact page: sodadome.com/contactme.php.

Hope I helped.