docs: match story docs and shell themes (#12399)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>
Co-authored-by: TJ Egan <tw15egan@gmail.com>

Author: 3 people

packages/react/.storybook/preview.js

@@ -14,6 +14,8 @@ import React from 'react';
 import { breakpoints } from '@carbon/layout';
 import { GlobalTheme } from '../src/components/Theme';
 
+import theme from './theme';
+
 export const globalTypes = {
   locale: {
     name: 'Locale',
@@ -87,6 +89,9 @@ export const parameters = {
   darkMode: {
     current: 'light',
   },
+  docs: {
+    theme,
+  },
   // Small (<672)
   // Medium (672 - 1056px)
   // Large (1056 - 1312px)