If you'd like to separate your website areas by main menu first levels (for example colourize all areas in a different colour) a body class outputting the first level mlid may help you (otherwise you can for example use context.module or things like that!):
Here's the snippet you may use:
function YOUR_THEME_preprocess_html(&$variables, $hook) {
$active_trail = menu_get_active_trail();
if (!empty($active_trail) && !empty($active_trail[1])) {
$first_level_mlid = $active_trail[1]['mlid'];
$variables['classes_array'][] = drupal_html_class('section-by-category-id-' . $first_level_mlid);
}
}
?>
