How to load custom CSS in a twig template?

I am using a twig template in my custom module (not theme folder) at this path: my_module/templates/mail-body.html.twig

I have problem attaching CSS with this template and need input how can I do this?

As per instructions on https://www.drupal.org/docs/8/creating-custom-modules/adding-stylesheets… page, I have used

{{ attach_library('my_module/mail-styling') }}

in my mail-body.html.twig file but still css is not loading.

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/4/feed