docs: add theme breaking changes to v5 upgrade guide

Author: johnleider

packages/docs/src/pages/en/getting-started/upgrade-guide.md

@@ -235,6 +235,27 @@ export default createVuetify({
 
 Theme colors now support transparency. `rgb(var(--v-theme-color))` will continue to work the same as before, but `rgba(var(--v-theme-color), 0.8)` should be changed to either `color-mix(in srgb, rgb(var(--v-theme-color)) 80%, transparent)` or `rgb(from rgb(var(--v-theme-color)) / 0.8)` when used with a transparent theme color.
 
+The theme system now uses `@vuetify/v0` under the hood. The consumer API (`useTheme`, `VThemeProvider`) is unchanged for most users.
+
+### Removed: `isDisabled`, `isSystem`, `styles`
+
+These refs are no longer available on the theme instance.
+
+### Runtime theme changes
+
+Direct assignment of new themes via `themes.value` is replaced by `register()`:
+
+```diff
+- theme.themes.value.custom = { dark: true, colors: { primary: '#ff5722' } }
++ theme.register({ id: 'custom', dark: true, colors: { primary: '#ff5722' } })
+```
+
+Mutating existing theme colors continues to work:
+
+```ts
+theme.themes.value.light.colors.primary = '#ff0000'
+```
+
 ## Components
 
 ### VBtn display