My custom CSS is not taken into account
This question has nothing to do with Drupal but with CSS order of precedence. If more than one identical CSS rules are found, the last one takes precedence.

As @leymannx pointed in their comment, your page is loading first your CSS file, then Bootstrap. Just move your Bootstrap into a new library as you just did with 'mymovies', then add a dependency to 'mymovies'. Declaring it as a dependency, it will load first the need libraries, then the library being called.

Edit - after saying you are using OnePage Zymphonies theme:

If you check 'onepage_zymphonies_theme.libraries.yml' you will see that the Bootstrap library is defined. Following my answer:

mymovies:
  version: 1.x
  css:
    theme:
      css/mymovies.css: {}
  dependencies:
      - onepage_zymphonies_theme/bootstrap