8
OR
In your news module code put this in somewhere, where it makes sense (i.e. after you have found topic id):
switch($xoopsOption['storytopic']) {
case '1': //if it is topic with id 1
$xoopsConfig['theme_set'] = "New Theme Name";
break;
case '2': //if it is topic with id 2
$xoopsConfig['theme_set'] = "Another theme name";
break;
}
I guess it is possible to hack into the topic manager to set a theme for each topic, but I'll leave you to try out this first.