Check if dark mode is enabled and toggle

Storybook components are wrapped using a decorator in the storybook config. When rendering, useDarkMode() will let the component know which theme (dark/light) to thoose.

packages/storybook/.storybook/config.js

@@ -1,10 +1,11 @@
 import React from 'react';
 import { addDecorator } from '@storybook/react';
-import { lightTheme } from '@backstage/theme';
+import { lightTheme, darkTheme } from '@backstage/theme';
 import { CssBaseline, ThemeProvider } from '@material-ui/core';
+import { useDarkMode } from 'storybook-dark-mode';
 
 addDecorator(story => (
-  <ThemeProvider theme={lightTheme}>
+  <ThemeProvider theme={useDarkMode() ? darkTheme : lightTheme}>
     <CssBaseline>{story()}</CssBaseline>
   </ThemeProvider>
 ));