You can use the following code. Asuming that the css file style.css
is inside the css folder of your theme:
function wpse_126614_enque( $hook ) {
if ( 'widgets.php' != $hook ) {
return;
}
$template_url = get_template_directory_uri();
wp_enqueue_style( 'admin-style-widget', $template_url . '/css/style.css' );
}
add_action( 'admin_enqueue_scripts', 'wpse_126614_enque' );