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: