Thanks for following up with us. Adding a style to the ruler's style menu does not set that style as the default for new pages. To set default content for new pages, you can access the Preferences menu and edit New Page Text under the General tab. When you create a new page, the contents and style of the New Page Text field should be added to that page.
If you have any other questions or run into issues with the Preferences menu, please let me know and I would be happy to help out!
I'm glad to hear that everything is working smoothly now. I'll see if we can make it clearer how styles work in the future to avoid this kind of confusion.
In particular, I should note that the name of a style doesn't impact how it can be used, so naming a style "Par défaut" should not make it the default style. Could you let me know if you read anywhere that naming the style that way should have an effect, or if you were also selecting another option which seemed like it would make that style the default style?