theming: add test that would fail at runtime

types/theming/theming-tests.tsx

@@ -2,11 +2,16 @@ import * as React from "react";
 import {
   channel,
   ContextWithTheme,
+  Theme,
   themeListener,
   ThemeProvider,
   withTheme
 } from "theming";
 
+// Typings currently accept non-plain-objects while they get rejected at runtime.
+// There exists currently no typing for plain objects.
+const runtimeErrorTheme: Theme = [];
+
 const customTheme = {
   color: {
     primary: "red",