Add support for multiple themes, i.e. a GitHub-like theme

[ooaklee]

SUMMARY

Currently, the leading portal only includes a single theme. Add functionality to allow users to specify the theme they want to use instead, i.e., a GitHub-like theme. If a referenced theme does not exist, we should default to the current theme and give a warning in the terminal.

Users should be able to specify the theme by using an input, e.g. theme: "github-light" and the default value of the theme input should be "default".

SUCCESS CRITERIA

• Add a new input theme that can be set to a theme name
  • We should only support "default" in the initial version and display a warning if any other theme is specified.
• Update the README to include a flow with the new theme input, mention that we currently only support "default" theme