[CORL-360] Embrace CSS Variables (#2312)

* fix: docz bug

* feat: implement css variables

* fix: adapt docz

* fix: webpack config

* fix: add ui l10n bundle to tests

src/core/client/admin/index.tsx

@@ -6,6 +6,9 @@ import EntryContainer from "./containers/EntryContainer";
 import { initLocalState } from "./local";
 import localesData from "./locales";
 
+// Import css variables.
+import "talk-ui/theme/variables.css";
+
 async function main() {
   const ManagedTalkContextProvider = await createManaged({
     initLocalState,