WP Enqueue style on all plug-in pages

If you want enqueue style on all admin pages You can use the following code

    function load_custom_wp_admin_style() {
        wp_enqueue_style( 'custom_wp_admin_css', plugins_url('css/style.css', __FILE__) );
}
add_action( 'admin_enqueue_scripts', 'load_custom_wp_admin_style' );

Or if you want enqueue style on specific pages, You can use the following code

function load_custom_wp_admin_style($hook) {
        if($hook != 'toplevel_page_myplugin' || $hook != 'widgets.php' || $hook != 'edit.php') {
                return;
        }
        wp_enqueue_style( 'custom_wp_admin_css', plugins_url('css/style.css', __FILE__) );
}
    add_action( 'admin_enqueue_scripts', 'load_custom_wp_admin_style' );