Adding theme editor field highlighting

[jasonnrosa]

Types of change

• [ ] Bug fix (change which fixes an issue)
• [ ] Enhancement (change which improves upon an existing feature)
• [X] New feature (change which adds new functionality)

Description

This adds the theme editor field highlighting feature to the boilerplate. This feature allows developers to attach selectors to their theme settings so that the theme setting highlights those elements when a user edits their theme settings (more information here). To do this, I used the built-in script that is noted in the documentation and then cleaned things up a bit manually based on some testing in the theme editor.

Relevant links

Fixes #501

Checklist

• [X] I have read the CONTRIBUTING document.
• [X] My code follows the style guide requirements.
• [X] I have thoroughly tested my change.

People to notify CC: @TheWebTech and @ajlaporte