Bug 1544026 - Add a tab color for the in-content.dark-mode mode. r=dao

Differential Revision: https://phabricator.services.mozilla.com/D27311

--HG--
extra : rebase_source : 8da67a543b83108e8b3dcf13df97ed6da4f758c8

toolkit/themes/shared/in-content/common.inc.css

@@ -109,6 +109,8 @@
 
     --in-content-category-text-selected: var(--blue-40);
     --in-content-link-color: var(--blue-40);
+
+    --in-content-tab-color: var(--in-content-page-color);
   }
 }
 }