Added toolbar icon for Dark mode controls in Storybook

.storybook/preview.js

@@ -1,19 +1,32 @@
-import React from "react";
+import React, { useState } from "react";
 import ItemProvider from '../src/components/ItemProvider';
 import { ThemeProvider } from "@material-ui/core/styles";
 import webqueueTheme from "../src/theme";
 
-const theme = webqueueTheme();
+const theme = webqueueTheme(false);
+
+export const globalTypes = {
+  theme: {
+    name: 'Theme',
+    description: 'Global theme for components',
+    defaultValue: 'false',
+    toolbar: {
+      icon: 'mirror',
+      // array of plain string values or MenuItem shape (see below)
+      items: ["light", "dark"],
+    },
+  },
+};
 
 export const decorators = [
   (Story) => (
     <ItemProvider>
       <Story />
     </ItemProvider>
   ),
-  (Story) => (
+  (Story, context) => (
     <ThemeProvider theme={theme}>
-      <Story />
+      <Story {...context} />
     </ThemeProvider>
   ),
 ];