To change the banner of the theme
find and change the code below in the THEME.css file which you can find in
C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\THEMES
In the code below the items in red are the ones I change to a) display a different image – create the image with 2 pixel width and the height to fit your banner and b) repeat the image horizontally – remember page loads it from left side.
after changing the code
1 – iisreset the sharepoint server
2- re-apply the theme
for changes to take effect.