lwa-modal-bg in html

I like the plugin very much.

But I have a doubt on how to apply ‘lwa-modal-bg’ in the html.
I want a bigger model box. I tried adding ‘lwa-modal-bg’ to the class with ‘lwa-modal’. But there was no change.

How can I update the fields in the <?php do_action(‘register_form’); ?>.
I have enabled password field and recaptcha in it. But I am not able to customize them.

https://www.ads-software.com/plugins/login-with-ajax/