Adjust styles to match the design system (#11118)

### What problem does this PR solve?

- Modify and adjust styles (CSS vars, components) to match the design
system
- Adjust file and directory structure of admin UI

### Type of change

- [x] Refactoring

web/src/pages/admin/layouts/root-layout.tsx

@@ -0,0 +1,7 @@
+import { Outlet } from 'umi';
+
+const AdminRootLayout = () => {
+  return <Outlet />;
+};
+
+export default AdminRootLayout;