docs: Update migration guide with ThemeProvider changes

documentation/guides/migrating-from-v7-to-v8.md

@@ -1,3 +1,40 @@
 # Migrating from v7 to v8
 
 Polaris v8.0.0 ([full release notes](https://github.com/Shopify/polaris-react/releases/tag/v8.0.0)) features a rebuild of the `Autocomplete` component, build changes, . This file describes all code updates required to stay up to date.
+
+## `ThemeProvider` changes
+
+The `ThemeProvider` has been deprecated in favor of the new `CustomProperties` component. As a result, a number of internal components using the `ThemeProvider` have been updated to use the `CustomProperties` component and adjusted their prop interfaces accordingly (such as: `AppProvider`, `Popover`, etc.).
+
+`polaris-react` no longer supports accepting and transforming a custom theme object to influence the component library. Polaris will now maintain a set of predefined color-schemes that meet the immediate needs of the admin and thus the following changes are required:
+
+```diff
+- import {ThemeProvider} from '@shopify/polaris-react';
++ import {CustomProperties} from '@shopify/polaris-react';
+
+const App = (props) => (
+-  <ThemeProvider theme={{colorScheme: 'dark'}}>
+-    {props.children}
+-  </ThemeProvider>
++  <CustomProperties colorScheme="dark">
++    {props.children}
++  </CustomProperties>
+)
+```
+
+As mentioned above, the `ThemeProvider` has been removed from the `AppProvider` and replaced with the `CustomProperties` component.
+
+With that said, the `AppProvider` no longer accepts a custom theme object to forward to the `ThemeProvider`. However, similar behavior is still optionally exposed by forwarding the `colorScheme` prop to the `CustomProperties` component:
+
+```diff
+import {AppProvider} from '@shopify/polaris-react';
+
+const App = (props) => (
+-  <AppProvider theme={{colorScheme: 'dark'}}>
+-    {props.children}
+-  </AppProvider>
++  <AppProvider colorScheme="dark">
++    {props.children}
++  </AppProvider>
+)
+```