See:
- https://robferguson.org/blog/2017/11/12/theming-your-ionic-3-app/
- https://robferguson.org/blog/2017/11/13/theming-your-ionic-3-app-part-2/
- https://robferguson.org/blog/2018/03/10/theming-your-ionic-3-app-the-colors-map/
- https://robferguson.org/blog/2018/04/09/theming-your-ionic-3-app-bespoke-svg-icons/