Add css style to theme in module
To do this, you need to create a *.libraries.yml file, with the path to your css file in it:

MODULE.libaries.yml:

star-field:
  version: Version
  css:
    theme:
      path/to/stylesheet.css: {}


Then you can attach that library to your element:

$element['#attached']['library'][] = MODULE/star-field;


This can be overridden in a theme if/when necessary, so this styling should be considered the default styling that the module creates.

More reading: https://www.drupal.org/docs/8/creating-custom-modules/adding-stylesheets-css-and-javascript-js-to-a-drupal-8-module