Add dark mode to storybook (#39656)

.storybook/manager.ts

@@ -1,7 +1,11 @@
 import { addons } from 'storybook/manager-api';
-
-import theme from './storybook-theme';
+import { themes } from 'storybook/theming';
 
 addons.setConfig({
-  theme,
+  theme: {
+    ...themes.normal,
+    brandTitle: 'Mastodon Storybook',
+    brandImage: 'logo.svg',
+    colorSecondary: '#6364FF',
+  },
 });

.storybook/storybook-theme.ts

@@ -1,7 +0,0 @@
-import { create } from 'storybook/theming';
-
-export default create({
-  base: 'light',
-  brandTitle: 'Mastodon Storybook',
-  brandImage: 'https://joinmastodon.org/logos/wordmark-black-text.svg',
-});