Add a custom class name to Wordpress body tag

Soution code for : Add a custom class name to Wordpress body tag

We can use the body_class filter, like so:

function my_plugin_body_class($classes) {
    $classes[] = 'foo';
    return $classes;
}

add_filter('body_class', 'my_plugin_body_class');

Although, obviously, our theme needs to call the corresponding body_class function.