storybook: fix default background

Author: tariknz

.storybook/preview.tsx

@@ -2,10 +2,8 @@ import type { Preview } from '@storybook/react-vite';
 import '../src/frontend/theme.css';
 
 const preview: Preview = {
-  parameters: {
-    backgrounds: {
-      default: 'dark',
-    },
+  initialGlobals: {
+    backgrounds: { value: 'dark' },
   },
 };