Hello @Francis Milhau

Hope you are doing well.

You can not use your custom css from a file inside your module to style index file for Odoo apps, however you can use bootstrap's own classes (supported in Odoo) for the same, other alternative is to use internal or in-line style.

Hope it will be helpful to you.

Thanks & Regards,
Email:odoo@aktivsoftware.com

Skype: kalpeshmaheshwari