6
           
            
                
     
    
    search for:
if ( isset( $HTTP_POST_VARS['xoops_theme_select'] ) && in_array( $HTTP_POST_VARS['xoops_theme_select'], $xoopsConfig['theme_set_allowed'] ) ) {
 $xoopsConfig['theme_set'] = $HTTP_POST_VARS['xoops_theme_select'];
 $HTTP_SESSION_VARS['xoopsUserTheme'] = $HTTP_POST_VARS['xoops_theme_select'];
 } elseif (isset($HTTP_SESSION_VARS['xoopsUserTheme']) && in_array($HTTP_SESSION_VARS['xoopsUserTheme'], $xoopsConfig['theme_set_allowed'])) {
 $xoopsConfig['theme_set'] = $HTTP_SESSION_VARS['xoopsUserTheme'];
 }
add after:
if ( isset( $HTTP_GET_VARS['usetheme'] ) && in_array( $HTTP_GET_VARS['usetheme'], $xoopsConfig['theme_set_allowed'] ) ) {
 $xoopsConfig['theme_set'] = $HTTP_GET_VARS['usetheme'];
 $HTTP_SESSION_VARS['xoopsUserTheme'] = $HTTP_GET_VARS['usetheme'];
 } elseif (isset($HTTP_SESSION_VARS['xoopsUserTheme']) && in_array($HTTP_SESSION_VARS['xoopsUserTheme'], $xoopsConfig['theme_set_allowed'])) {
 $xoopsConfig['theme_set'] = $HTTP_SESSION_VARS['xoopsUserTheme'];
 }
now you can add a html redirect site
with url: 
http://www.yourdomain.de/index.php?usetheme=THEMENAME :)
Now you can get different themes on each site