System menu blocks don't get the same styling as menu module blocks

In D7, the system module is in charge of the following menu blocks: Navigation, Management, User menu, Main menu, Secondary menu

This means that in order to apply the same styling to all menu blocks, you need to use the following CSS selector:

.block-menu,
#block-system-navigation,
#block-system-management,
#block-system-user-menu,
#block-system-main-menu,
#block-system-secondary-menu {
  // styling
}

This is just awful, awful experience for themers and non-obvious. Fortunately, its easy to add the "block-menu" class back to those 5 special case system-provided menu blocks.

function FOO_preprocess_block(&$variables) {
   // System menu blocks should get the same class as menu module blocks.
  if ($variables['block']->module == 'system' && in_array($variables['block']->delta, array_keys(menu_list_system_menus()))) {
    $variables['classes_array'][] = 'block-menu';
  }
}

[above code edited after comment #4 was made.]

We should definitely do that to make things easier for themers and to make our stylesheets lighter weight (re: performance).