Bug 1800189 - Set with system dark theme the tab line color. r=aleca

Differential Revision: https://phabricator.services.mozilla.com/D161928

--HG--
extra : amend_source : a3b154d54d46482e7fee35f5b3e24f69cdd01965

mail/themes/shared/mail/variables.css

@@ -172,6 +172,7 @@ toolbar[brighttext] {
 
   :root[lwt-default-theme-in-dark-mode] {
     --lwt-selected-tab-background-color: #2a2a2e !important;
+    --lwt-tab-line-color: #0a84ff !important;
     --sidebar-highlight-background-color: #0a84ff;
   }