Article 2 de Fabrice

YES, « there’s a (SIMPLE) way to make (CSS) style rules work in the main stylesheet of the theme. » and this way will also be visible while using the full-site-editor.

The answer the girls gave is a bit misleading, since they never explained where are « the custom CSS features that are built into WordPress ». Which lead me to hours lost searching for what they meant.
The section for CSS styles is located directly in « Twenty Twenty-Four » Theme Editor, just go:

1. Appearance (themes);
2. Editor (site-editor);
3. Left Menu -> Styles (wp_global_styles);
4. There’s an Eye and a Pencil icons, click the pencil (edit);
5. To the left, at the bottom you’ll find « Additional CSS » or at the top inside the three dots button.

You can also go directly to « /wp-admin/site-editor.php?path=%2Fwp_global_styles&canvas=edit » and look for step 5.

Hope this helps many newbies with this template like me.