I would like the questions to be displayed the same way that on this bbpress forum.
On these forums the developer removed the content holding the labels and also moved the post date and id number in a different area.
Robin linked you to a plugin that enables you to show the lead topic, which this site does, so you have that. If you do not want to use the plugin, or you are really just using it to just show the lead topic, you can also use this custom php snippet that is on this site.
https://codex.bbpress.org/bbp_show_lead_topic/
I still do not find a way to add a border around the topic (and not the replies) or find a way to make it look different from the answers.
You can try some custom CSS like this. It won’t look as good as this site because of how its designed as I stated above.
.single-topic .topic {
border: 2px solid black;
}I do not understand why the line in blue with “créateur” and “sujet” appears above and below the main topic.
Thats normal, its how the lead topic is supposed to be displayed by default.