Set the initial Storybook theme to light (#711)

This fixes the inconsistency if user's OS/Browser theme is set to Dark mode.

packages/storybook/.storybook/config.js

@@ -1,14 +1,21 @@
 import React from 'react';
-import { addDecorator } from '@storybook/react';
+import { addDecorator, addParameters } from '@storybook/react';
 import { lightTheme, darkTheme } from '@backstage/theme';
 import { CssBaseline, ThemeProvider } from '@material-ui/core';
 import { useDarkMode } from 'storybook-dark-mode';
 import { Content } from '@backstage/core';
 
-addDecorator(story => (
+addDecorator((story) => (
   <ThemeProvider theme={useDarkMode() ? darkTheme : lightTheme}>
     <CssBaseline>
       <Content>{story()}</Content>
     </CssBaseline>
   </ThemeProvider>
 ));
+
+addParameters({
+  darkMode: {
+    // Set the initial theme
+    current: 'light',
+  },
+});