In includes/page_header.php find:
$site_template->register_vars(array(
Insert ABOVE:$file = get_included_files();
$file = get_file_name(basename($file[0]));
$list = array("categories", "details", "index", "member", "postcards", "register", "search", "top");
$array = array();
foreach ($list as $name)
{
$array[$name] = ($name == $file);
}
$site_template->register_vars($array);
After that you can use in any template: {if index}...{if index} for home page , {if details}..{endif details} for details.php page, so on (check the $list array for the available tag names)