Bootstrap sub-theme override CSS

I just set up an new Drupal 8 website. I was used to use CSS injector on Drupal 7, but since the module is not ready yet, and I thought I’d better start with subthemes, I tried to create a new Boostrap subtheme. I used the CDN method, activated it in my admin panel, but when I try to override CSS by creating CSS styles in css/style.css, nothing work. In Drupal 7 with CSS injector, my main difficulty was to find the proper class. So I tried a lot of different possibilities, copied/paste CSS from original files in Bootstrap. Nothing works.

Have anybody already tried to create a sub-theme with Bootstrap? Did overriding CSS work?

I looked at the CSS file link, it’s loading lumen theme nicely. I tried to change h2 color in this file: themes/bootstrap/css/3.3.5/overrides-lumen.min.css and it works…Clearly, seems that style.css is not used…