[docs] Improve migration guide for @material-ui/styles (mui#26552)

Author: mnajdova

docs/src/pages/guides/migration-v4/migration-v4.md

@@ -1834,3 +1834,15 @@ If you are using the utilities from `@material-ui/styles` together with the `@ma
 -import { ThemeProvider } from '@material-ui/styles';
 +import { ThemeProvider } from '@material-ui/core/styles';
 ```
+
+#### Default theme (TypeScript)
+
+The `@material-ui/styles` package is no longer part of `@material-ui/core/styles`. If you are using `@material-ui/styles` together with `@material-ui/core` you need to add a module augmentation for the `DefaultTheme`.
+
+```ts
+import { Theme } from '@material-ui/core/styles';
+
+declare module '@material-ui/styles' {
+  interface DefaultTheme extends Theme {}
+}
+```