message: Fix outdated custom theme docs
diff: @@ -53,7 +53,7 @@ To add a custom theme to your Backstage app, you pass it as configuration to
  For example, adding the theme that we created in the previous section can be
  done like this:
  -```ts
  +```tsx
  import { createApp } from '@backstage/app-defaults';
  const app = createApp({
  @@ -63,7 +63,11 @@ const app = createApp({
  id: 'my-theme',
  title: 'My Custom Theme',
  variant: 'light',
  - theme: myTheme,
  + Provider: ({ children }) => (
  + <ThemeProvider theme={myTheme}>
  + <CssBaseline>{children}</CssBaseline>
  + </ThemeProvider>
  + ),
  }]
  })
  ```
  @@ -162,7 +166,7 @@ wouldn't be enough to alter the `box-shadow` property or to add css rules that
  aren't already defined like a margin. For these cases you should also create an
  override.
  -```ts
  +```tsx
  import { createApp } from '@backstage/core-app-api';
  import { BackstageTheme, lightTheme } from '@backstage/theme';
  /**
  @@ -187,14 +191,7 @@ export const createCustomThemeOverrides = (
  };
  };
  -const app = createApp({
  - apis: ...,
  - plugins: ...,
  - themes: [{
  - id: 'my-theme',
  - title: 'My Custom Theme',
  - variant: 'light',
  - theme: {
  +const customTheme: BackstageTheme = {
  ...lightTheme,
  overrides: {
  // These are the overrides that Backstage applies to `material-ui` components
  @@ -202,7 +199,20 @@ 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={customTheme}>
  + <CssBaseline>{children}</CssBaseline>
  + </ThemeProvider>
  + ),
  }]
  });
  ```
  