docs(migration): add info about removing old light palette colors (#3586)

* docs(migration): add info about removing old light palette colors

* Update docs/updating/8-0.md

Co-authored-by: Liam DeBeasi <[email protected]>

* lint

---------

Co-authored-by: Liam DeBeasi <[email protected]>

Author: averyjohnston

docs/updating/8-0.md

@@ -74,6 +74,28 @@ npm install @ionic/core@next
 
 The following changes are not required to update to Ionic 8 as your application will continue to work. However, we recommend making the following changes to ensure you can use the new features in Ionic 8.
 
+### Light Theme
+
+Previous versions shipped a set of default color variables for the light theme:
+
+```css
+/** Ionic CSS Variables **/
+:root {
+  /** primary **/
+  --ion-color-primary: #3880ff;
+  --ion-color-primary-rgb: 56, 128, 255;
+  --ion-color-primary-contrast: #ffffff;
+  --ion-color-primary-contrast-rgb: 255, 255, 255;
+  /* ... */
+}
+```
+
+In Ionic Framework version 8, the default color palette is included as long as `core.css` is imported. Removing the old color variables ensures that the latest palette is not overwritten.
+
+Developers who are customizing this color palette can continue to keep the custom variables values, but any of the variables that use the default values should be removed.
+
+You can read more about the new color palette in the [Ionic v8 announcement](https://ionic.io/blog/announcing-the-ionic-8-beta).
+
 ### Dark Theme
 
 In previous versions, it was recommended to define the dark theme in the following way: