chore: add note about scoped default theme for elements (#2287)

Author: jonas-jonas

docs/elements/guides/02_theming.mdx

@@ -70,3 +70,9 @@ function MyApp({ Component, pageProps }) {
 }
 export default MyApp
 ```
+
+## Using the default theme with custom components
+
+When building custom components, you can still leverage the default theme by using the CSS variables defined in Ory Elements. The
+default theme's styles are scoped to the `.ory-elements` CSS class. This means that if you wrap your custom components in a
+container with this class, they will inherit the default theme styles.