Add Modern Usage Example

[tylerkrupicka]

One area where this project struggles with documentation is useful examples. We document bits and pieces of the PostCSS configuration required to use this project, but it doesn't really give a full picture for somebody new to PostCSS in general.

I would like to create a folder in the repository called examples and in it add a folder called modern which has a simple app using the modern theme configuration and PostCSS.

This would require:

• Creating the folders.
• Create a sample application using Create React App, Webpack, or similar.
• Set up PostCSS using this plugin (with comments in the PostCSS config).
• Create a simple page that contains a button that has themes for different products (maybe TT and QB), and a light and dark mode.
• Toggles to switch between themes and color schemes to show it working.

I think this would greatly simplify setup for people using this project or PostCSS for the first time.