Hi Ben,
Thank you for your response. Unfortunately, the width: auto !important breaks our css flow (and we don’t like !important 
)
If we comment the line in the wp-content/plugins/bb-plugin/css/fl-builder-layout-responsive.css file, all is fine. But if we need to update BB, it will be crushed.
To answer your question : no, we use BB plugin with a unique custom theme for one of our clients. This theme is based on the starter theme Underscores.