I add a CSS file (mymovies.css) to a custom form (SeenMovies) in a custom module (eric) with this code I inserted at the end of buildForm

$form['#attached']['library'][] = 'eric/mymovies';

and this code in eric.libraries.yml

mymovies:
  version: 1.x
  css:
    theme:
      css/mymovies.css: {}

But my css doesn't override bootstrap.min.css as you can see in Firefox inspector:

How can I fix that?