docs: storybook themes (#5855)

Author: lisalupi

.storybook/preview.tsx

@@ -10,6 +10,7 @@ import DocsContainer from './components/DocsContainer'
 import Page from './components/Page'
 import { dark, light } from './storybookThemes'
 import '@ultraviolet/fonts/fonts.css'
+import { withThemeByClassName } from '@storybook/addon-themes'
 
 import { scan } from "react-scan"
 
@@ -140,6 +141,21 @@ const withThemeProvider = (Story: StoryFn, context: { globals: { theme: string }
   )
 }
 
+
+const preview: Preview = {
+  decorators: [
+    withThemeByClassName({
+      themes: {
+        light: '',
+        dark: 'dark',
+        darker: "darker"
+      },
+      defaultTheme: 'light',
+    }),
+  ],
+};
+ 
+
 const decorators = [
   (Story: StoryFn, context: { globals: { theme: string } }) => {
     const theme = context.globals.theme || "light"
@@ -171,5 +187,6 @@ const decorators = [
 export default {
   parameters,
   decorators,
+  preview,
   tags: ['autodocs']
 } satisfies Preview