How to use custom CSS (not inline) inside WordPress just for a template engine?

I try to use CSS code only for the specific use like newsletter and template engine.
If I add usual function, it will be seen also inside regular theme if a custom plugin is activated.

An example:

<?php

function enqueue_custom_style()
{
    wp_enqueue_style('custom-style', 'styles_newsletter.css', array(), '1.0');
}
?>

How to avoid this?

The recommended way, I believe, is to create a child theme.

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