There already exists a feature request for this, but in the meantime, here is an idea (untested!) for a hack to accomplish this:
In lib-common.php
in function COM_siteHeader
, the URL for the stylesheet to use is set like this:
$header->set_var('css_url', $_CONF['layout_url'] . '/style.css');
You could change this to switch stylesheets based on the current topic like this:
if (!empty ($topic)) {
$header->set_var('css_url', $_CONF['layout_url'] . '/' . $topic . '.css');
} else {
$header->set_var('css_url', $_CONF['layout_url'] . '/style.css');
}
You would then need one stylesheet per topic that has the same name as the topic ID you gave your topics in the Admin topic editor.
Hope that helps (and works ...)
bye, Dirk