Je devrais faire mes recherches avant de poster haha, j'ai trouvé https://join-lemmy.org/docs/administration/theming.html
Lemmy uses Bootstrap v4, and very few custom css classes, so any bootstrap v4 compatible theme should work fine. Use a tool like bootstrap.build to create a bootstrap v4 theme. Export the bootstrap.min.css once you're done, and save the _variables.scss too.
If you installed Lemmy with Docker, save your theme file to ./volumes/lemmy-ui/extra_themes. For native installation (without Docker), themes are loaded by lemmy-ui from ./extra_themes folder. A different path can be specified with LEMMY_UI_EXTRA_THEMES_FOLDER environment variable.
After a theme is added, users can select it under /settings. Admins can set a theme as site default under /admin.