Add a class to every page in Drupal 7

add_class_d7

/**
 * Implements hook_preprocess_htm().
 */
function THEME_preprocess_html(&$variables) {
    $path = drupal_get_path_alias();
    $aliases = explode('/', $path);
    foreach($aliases as $alias) {
      $variables['classes_array'][] = 'page-alias-' . drupal_clean_css_identifier($alias);
    }
}