import dark mode from DSFR and setup as system

app/javascript/entrypoints/main.css

@@ -1,5 +1,6 @@
 @import '@gouvfr/dsfr/dist/core/core.css';
 @import '@gouvfr/dsfr/dist/utility/utility.css';
+@import '@gouvfr/dsfr/dist/scheme/scheme.css';
 
 /* These base components may be dependencies of other components */
 @import '@gouvfr/dsfr/dist/component/link/link.css';

app/views/layouts/application.html.haml

@@ -1,5 +1,5 @@
 !!! 5
-%html{ lang: html_lang, class: yield(:root_class) }
+%html{ lang: html_lang, data: { fr_scheme: 'system' }, class: yield(:root_class) }
   %head
     %meta{ "http-equiv": "Content-Type", content: "text/html; charset=UTF-8" }
     %meta{ "http-equiv": "X-UA-Compatible", content: "IE=edge" }