How to add a class to meta box

Question

When adding a metabox, i.e.:

add_meta_box( 
    'metabox_id',
    'Metabox Title',
    'my_metabox_callback',
    'page',
    'normal',
    'low', 
    array( 'foo' => $var1, 'bar' => $var2)
);

how do I add a class to it for css styling? I'd like to avoid having to call each id in the style rule for metaboxes that contain elements having the same style.

Answer 1

Just add the class to the markup you are generating in the callback.

Answer 2

Let's say you want a class for the excerpt box. Then you can do:

add_filter('postbox_classes_post_postexcerpt','add_metabox_classes');

function add_metabox_classes($classes) {
    array_push($classes,'another_class');
    return $classes;
}

With this method you need to add a filter for every box u need to add a class for. The filter is applied in the function postbox_classes in wp-admin/includes/post.php

In general the hook is postbox_classes_{$page}_{$id} where $page is the page identifier (e.g. 'post' for posts and (presumably) 'custom-post-type' for posts of type 'custom-post-type'). $id refers to the ID of the metabox, automatically assigned for 'default' metaboxes or specified in add_meta_box.

DOCS: https://developer.wordpress.org/reference/hooks/postbox_classes_page_id/