Feature Overview (aka. Goal Summary)

Provide a way for a dynamic plugin to export a theme provider function.

Provide a method to configure the frontend to supply a theme provider function exported from a dynamic plugin to be picked up by the createApp API.

Expose the configuration items needed to display the theme properly to the user such as:

id - the internal ID of the theme
title - the user visible name of the theme
icon - the icon that should be displayed next to the theme name
variant - the variation of the theme either "light" or "dark"
importName - the name of the exported theme provider function

So for example a theme configuration could look something like:

dynamicPlugins: 
  frontend: 
    internal.backstage-plugin-test-backstage-theme: 
      appIcons: 
        - name: lightIcon
          importName: LightIcon
        - name: darkIcon
          importName: DarkIcon
      themes: 
        - id: light  # Using 'light' overrides the app-provided light theme
          title: Test Light Theme
          variant: light
          icon: lightIcon
          importName: lightThemeProvider
        - id: dark  # Using 'dark' overrides the app-provided dark theme
          title: Test Dark Theme
          variant: dark
          icon: darkIcon
          importName: darkThemeProvider