How can I add a new CSS class to a widget?

I want to assign a new CSS class to default widget <ul> in Page/Post creating page in WordPress. How can I do this?

See the image below.

Don’t edit your parent theme’s functions.php file.

register_sidebar( array(
    'name'          => __( 'Sidebar name', 'theme_text_domain' ),
    'id'            => 'unique-sidebar-id',
    'description'   => '',
    'class'         => 'add class here',
    'before_widget' => '<li id="%1$s" class="widget %2$s">',
    'after_widget'  => '</li>',
    'before_title'  => '<h2 class="widgettitle">',
    'after_title'   => '</h2>'
) );

Copy it over to your child theme’s functions.php and make the changes there, or simply register a new widget.

You could then hook it in from your child theme’s functions.php like this:

add_action( 'your_hook', 'new_widget' );
function new_widget() {
    if ( is_your_conditional_tag() && is_active_sidebar( 'new-widget' ) ) { 
        dynamic_sidebar( 'new-widget', array(       
            'before' => '<div class="new-widget">',
            'after' => '</div>'
        ) );
    }
}

You can also add more classes to the dynamic_sidebar function.