message: update docs to use ThemeProvider for custom theme overrides
diff: @@ -89,7 +89,7 @@ import {
  shapes,
  } from '@backstage/theme';
  -const customTheme = createTheme({
  +const myTheme = createTheme({
  palette: {
  ...lightTheme.palette,
  primary: {
  @@ -200,14 +200,7 @@ export const createCustomThemeOverrides = (
  };
  };
  -const app = createApp({
  - apis: ...,
  - plugins: ...,
  - themes: [{
  - id: 'my-theme',
  - title: 'My Custom Theme',
  - variant: 'light',
  - theme: {
  +const overriddenTheme = {
  ...lightTheme,
  overrides: {
  // These are the overrides that Backstage applies to `material-ui` components
  @@ -215,7 +208,20 @@ const app = createApp({
  // These are your custom overrides, either to `material-ui` or Backstage components.
  ...createCustomThemeOverrides(lightTheme),
  },
  - },
  +};
  +
  +const app = createApp({
  + apis: ...,
  + plugins: ...,
  + themes: [{
  + id: 'my-theme',
  + title: 'My Custom Theme',
  + variant: 'light',
  + Provider: ({ children }) => (
  + <ThemeProvider theme={overriddenTheme}>
  + <CssBaseline>{children}</CssBaseline>
  + </ThemeProvider>
  + ),
  }]
  });
  ```
  