Actually I did this with 2 different CSS and Dirk's help like this in the header.thtml:
Text Formatted Code
<link rel="stylesheet" type="text/css" href=
<?php
global $_CONF, $topic;
if ($topic == 'topicB') {print $_CONF['layout_url'] . '/topicB.css';}
else {print $_CONF['layout_url'] . '/style.css';}?>
title="{theme}">
It says mainly if the chosen topic is "topicB" then chose the css topicB.css
Obviously topicB.css contains a background pic which is different from style.css. I deleted the usual logo.gif in the header.thtml.
Text Formatted Code
.header-logobg-container-inner {
background:url(images/logo_topicB.gif);
background-repeat:no-repeat;
/*background:url(images/header-bg.png) #5A94D6;*/
border-bottom:1px solid #CCCCCC;
}
If you also want to change the site slogan in the header.thtml:
Text Formatted Code
<?php global $_CONF, $topic;
if ($topic == 'topicB') {print 'site slogan topicB';} else {print '{site_slogan}';}?>