Part 8 - Highlighting Tapped or Clicked UI Elements

- In styles.css, add a tappable-active class definition for tapped or
clicked list item links. The class simply highlights the item with a
blue background:
- In main.js, define a registerEvents() function inside the app object.
Add a the tappable_active class to the selected (tapped or clicked) list
item:
- Invoke the registerEvents() function from within the app object’s
initialize() function.