Add theme configuration support to ApolloServerPluginLandingPageLocalDefault

Feature Request

Is your feature request related to a problem? Please describe.

Currently, ApolloServerPluginLandingPageProductionDefault supports theme configuration (light/dark mode) via the document option, but ApolloServerPluginLandingPageLocalDefault does not offer this capability.

Describe the solution you'd like

I would like ApolloServerPluginLandingPageLocalDefault to support the same document configuration option as ApolloServerPluginLandingPageProductionDefault:

Example usage:

plugins: [
  ApolloServerPluginLandingPageLocalDefault({
    initialState: {
      displayOptions: {
        theme: 'dark', // or 'light'
      }
    },
  }),
]

Describe alternatives you've considered

1. Using Production Landing Page in Development: I specifically do not want to rely on production landing page as it require to login to studio apollo graphql, while I only use this page for local development.

Additional context

• The production landing page already has this feature implemented, suggesting the underlying infrastructure exists
• This appears to be a straightforward feature parity issue rather than a fundamental architectural limitation